Assurance, also known as the Witness of the Spirit, is a Protestant Christian doctrine that states that the inner witness of the Holy Spirit allows the Christian disciple to know that they are justified.   Based on the writings of St. Augustine of Hippo, assurance was historically a very important doctrine in Lutheranism and Calvinism, and remains a distinguishing doctrine of Methodism and Quakerism though there are differences among these Christian traditions.   Hymns that celebrate the witness of the Holy Spirit, such as " Blessed Assurance" are sung in Christian liturgies to celebrate the belief in assurance.
John Wesley believed that all Christians have a faith which implies an assurance of God's forgiving love, and that one would feel that assurance, or the "witness of the Spirit". This understanding is grounded in Paul's affirmation, "...ye have received the Spirit of adoption, whereby we cry Abba, Father. The same Spirit beareth witness with our spirits, that we are the children of God..." ( Romans 8:15-16, Wesley's translation). This experience was mirrored for Wesley in his Aldersgate experience wherein he "knew" he was loved by God and that his sins were forgiven.
- "I felt my heart strangely warmed. I felt I did trust in Christ, Christ alone for salvation, and an assurance was given me that He had taken my sin, even mine." — from Wesley's Journal 
Early in his ministry Wesley had to defend his understanding of assurance. In 1738 The Reverend Arthur Bedford had published a sermon in which he misquoted Wesley's teachings. Bedford had misunderstood Wesley as saying that a Christian could be assured of persevering in a state of salvation, the Reformed view.
In a letter dated September 28, 1738, Wesley wrote, "The assurance of which I alone speak I should not choose to call an assurance of salvation, but rather (with the Scriptures), the assurance of faith. . . . [This] is not the essence of faith, but a distinct gift of the Holy Ghost, whereby God shines upon his own work, and shows us that we are justified through faith in Christ...The 'full assurance of faith' ( Hebrews 10.22) is 'neither more nor less than hope; or a conviction, wrought in us by the Holy Ghost, that we have a measure of the true faith in Christ..'" 
The full assurance of faith taught by Methodists is the Holy Spirit's witness to a person who has been regenerated and entirely sanctified.  This full assurance of faith "excludes all doubt and fear since the heart has now been perfected in love", consistent with a Wesleyan–Arminian interpretation of 1 John 4:18, which proclaims "There is no fear in love; but perfect love casteth out fear: because fear hath torment. He that feareth is not made perfect in love." (KJV)  John Wesley emphasized that this is not an assurance about the future, but about the present state of the believer (Methodist theology teaches that apostasy can occur through sin or a loss of faith).  Believers can be assured that they are the adopted children of God and will be with Him for eternity if they continue in holiness by trusting in Christ and obeying God's commandments in this life. 
The witness of the Spirit is that inward impression wrought on the soul whereby the Spirit of God immediately and directly assures our spirit that Bible conditions are met for salvation and the work of grace is complete in the soul (Romans 8:16). Therefore, the Spirit bears witness to both the salvation of the sinner and the sanctification of the believer (Hebrews 10: 14-15; (I John 5:10). 
The witness of the Spirit is that inward impression wrought on the soul, whereby the Spirit of God immediately and directly assures our spirit that the Bible conditions are met for salvation and the work of grace is complete in the soul (Romans 8:15, 16). Therefore none should think they are either saved or sanctified until the Spirit of God has added His testimony (1 John 5:10). And if we take car to all with God, and not grieve the Holy Ghost, we shall have an abiding testimony (Ephesians 4:30). 
The Central Yearly Meeting of Friends, a Holiness Quaker denomination, teaches in reference to the experiences of the New Birth and Perfection "that the Spirit of God gives to each born again person an inward witness that he is truly a child of God and to each truly sanctified person a witness that he is entirely sanctified."  Quakers hold that the "witness of the Spirit is nothing more than the communication and assurance of God through the Spirit to the inward consciousness of the seeking and the believing soul that he has received that which he desired of God, that God has both hear the prayer and performed His work of grace in the heart (Rom 8;16; I Jn. 5:14, 15)." 
Baptists teach that a "person is born again when he/she repents of his/her sins and asks Jesus to forgive him/her and trust Jesus to serve him him/her."  Those who have been born again, according to Baptist teaching, know that they are "a child of God because the Holy Spirit witnesses to them that they are." 
Lutheranism accepts monergism, which states that salvation is by God's act alone, and rejects the teaching that humans in their fallen state have a free will concerning spiritual matters.  Lutherans believe that although humans have free will concerning civil righteousness, they cannot work spiritual righteousness without the Holy Spirit, since righteousness in the heart cannot be wrought in the absence of the Holy Spirit.  Lutherans believe that the elect are predestined to salvation.  According to Lutheranism, Christians should be assured that they are among the predestined.  Lutherans believe that all who trust in Jesus alone can be certain of their salvation, for it is in Christ's work and his promises in which their certainty lies.  However, they disagree with those that make predestination the source of salvation rather than Christ's suffering, death, and resurrection. Unlike Calvinists, Lutherans do not believe in a predestination to damnation.  Instead, Lutherans teach eternal damnation is a result of the unbeliever's sins, rejection of the forgiveness of sins, and unbelief, all of which occur when God chooses not to positively intervene during the unbeliever's lifetime.  The central final hope of the Christian is "the resurrection of the body and the life everlasting" as confessed in the Apostles' Creed, but Lutherans also teach that, at death, Christian souls are immediately taken into the presence of Jesus in heaven,  where they await this bodily resurrection and the second coming of Jesus on the Last Day. 
Reformed Christianity (Calvinism) teaches that believers may have assurance of their salvation especially through the work of the Holy Spirit and also by looking at the character of their lives.[ citation needed] The idea that because good works necessarily result from true faith one can gain assurance by observing evidences of faith in their life is called the practical syllogism.  If they believe God's promises and seek to live in accord with God's commands, then their good deeds done in response with a cheerful heart provide proof that can strengthen their assurance of salvation against doubts. This assurance is not, however, a necessary consequence of salvation, and such assurance may be shaken as well as strengthened. 
...infallible assurance doth not so belong to the essence of faith but that a true believer may wait long and conflict with many difficulties before he be partaker of it: yet, being enabled by the Spirit to know the things which are freely given him of God, he may, without extraordinary revelation, in the right use of ordinary means, attain thereunto. And therefore it is the duty of everyone to give all diligence to make his calling and election sure; that thereby his heart may be enlarged in peace and joy in the Holy Ghost, in love and thankfulness to God, and in strength and cheerfulness in the duties of obedience, the proper fruits of this assurance...
Additionally, the Augustinian doctrines of grace regarding predestination are taught in the Reformed churches primarily to assure believers of their salvation since the Calvinist doctrines emphasize that salvation is entirely a sovereign gift of God apart from the recipient's choice, deeds, or feelings (compare perseverance of the saints).
Anabaptists who belong to New Order Amish communities teach the belief in assurance—"that one can know the state of his soul while on earth".  This differs from the Old Order Amish understanding, who teach a "living hope" of salvation.  Obedience to Jesus and a careful keeping of the Ten Commandments, in addition to loving one another and being at peace with others are seen as "earmarks of the saved". 
The Catholic Church teaches that an infallible certitude of final salvation, as supposed in Calvinism, is not a usual experience, as seen in the sixteenth canon of the sixth session of the Council of Trent:
"If any one saith, that he will for certain, of an absolute and infallible certainty, have that great gift of perseverance unto the end,-unless he have learned this by special revelation; let him be anathema." 
In critiquing the Reformed doctrine of the assurance of salvation, prominent Catholic apologist Robert Sungenis notes a complication of the doctrine as it relates to the historic Protestant doctrine of Sola fide:
"The burden is on the Reformed position because it says that a person can live his whole life thinking that he is justified by faith alone and yet come to the point in time where he stands at the judgment seat of God and finds out that he or she did not have the works that qualified the faith to be justifying faith and therefore God would say, 'I’m sorry, you were never justified in the first place.' So, if there’s anyone who lives under a cloud of terror, it’s the Reformed Protestant because he or she never knows if he or she (or God) did the proper works in order to qualify the faith that he or she needs for justification. And this is especially important because the Reformed position says that works can never enter into the faith that procures the justification because works are all in sanctification. So, if works can never enter into the faith that one needs for justification how can they ever qualify the faith that one needs for justification? So, now there is a double-dilemma." 
Catholics recognize that a certainty of faith is ascribed to St. Paul (2 Cor 12,9) and speculate that the Virgin Mary also probably possessed it. Jesus Christ as man, however, did not need to believe since he knew it. Ludwig Ott argues that a high moral, human certainty of having sanctifying grace is possible, on the grounds that one is not conscious of an unforgiven grave sin, but by no means faith which is believing with divine certainty  and that with some probability one can locate positive signs of predestination, which does not mean that their lack be a sign of reprobation: He lists persistent action of the virtues recommended in the Eight Beatitudes, frequent Communion, active charity, love for Christ and the Church and devotion to the Blessed Virgin.  Moreover, and especially, a Catholic can, and should, have certain  hope for eternal salvation, which does not rest chiefly on a grace already received, but rather on prospective future forgiveness by God's omnipotence and mercy. The point in question is that however certain, the hope must retain its proper name and not be confused with faith. If together with a determination for sin, this hope is in danger of giving way to presumption. 
In the Catholic tradition, a close equivalent to a doctrine of assurance has been a doctrine of final perseverance.  Compliance with First Friday Devotions has sometimes been taught as a means to final perseverance.
- Yates, Arthur S. (2015). The Doctrine of Assurance: With Special Reference to John Wesley. Wipf and Stock Publishers.
Writing to Arthur Bedford on 4th August 1738, Wesley says: 'That assurance of which alone I speak, I should not choose to call an assurance of salvation, but rather (with the Scriptures) the assurance of faith. . . . I think the Scriptural words are ...
- Guidebook of the Emmanuel Association of Churches. Logansport: Emmanuel Association. 2002. p. 9.
- Manual of Faith and Practice of Central Yearly Meeting of Friends. Central Yearly Meeting of Friends. 2018. p. 28.
- The discussion of Wesley's understanding of assurance is a revision of information presented on the website "Days of Wesley", copyright 2004, Days of Wesley, Conrad Archer, Entry on Assurance Archived 2005-01-22 at the Wayback Machine.
- Abraham, William J.; Kirby, James E. (2009). The Oxford Handbook of Methodist Studies. Oxford University Press. ISBN 9780191607431.
- "Holiness". The Witness of the Spirit. Pilgrim Nazarene Church. 2017. Retrieved 6 June 2018.
- Longwe, Hany (2011). Christians by Grace—Baptists by Choice: A History of the Baptist Convention of Malawi. African Books Collective. p. 429. ISBN 978-99960-27-02-4.
- See Augsburg Confession, Article XVIII: Of Free Will
- 1 Cor. 2:14, 12:3, Rom. 8:7, Martin Chemnitz, Examination of the Council of Trent: Vol. I. Trans. Fred Kramer, St. Louis: Concordia Publishing House, 1971, pp. 409-53, "Seventh Topic, Concerning Free Will: From the Decree of the Sixth Session of the Council of Trent".
- Augsburg Confession, Article 18, Of Free Will Archived 2008-09-15 at the Wayback Machine.
- Acts 13:48, Eph. 1:4–11, Epitome of the Formula of Concord, Article 11, Election, Mueller, J.T., Christian Dogmatics. St. Louis: Concordia Publishing House, 1934. pp. 585-9, section "The Doctrine of Eternal Election: 1. The Definition of the Term", and Engelder, T.E.W., Popular Symbolics. St. Louis: Concordia Publishing House, 1934. pp. 124-8, Part XXXI. "The Election of Grace", paragraph 176.
- 2 Thess. 2:13, Mueller, J.T., Christian Dogmatics. St. Louis: Concordia Publishing House, 1934. pp. 589-593, section "The Doctrine of Eternal Election: 2. How Believers are to Consider Their Election, and Engelder, T.E.W., Popular Symbolics. St. Louis: Concordia Publishing House, 1934. pp. 127-8, Part XXXI. "The Election of Grace", paragraph 180.
- Rom. 8:33, Engelder, T.E.W., Popular Symbolics. St. Louis: Concordia Publishing House, 1934. pp. 127-8, Part XXXI. "The Election of Grace", paragraph 179.
- 1 Tim. 2:4, 2 Pet. 3:9, Epitome of the Formula of Concord, Article 11, Election, and Engelder's Popular Symbolics, Part XXXI. The Election of Grace, pp. 124-8.
- Hos. 13:9, Mueller, J.T., Christian Dogmatics. St. Louis: Concordia Publishing House, 1934. p. 637, section "The Doctrine of the Last Things (Eschatology), part 7. "Eternal Damnation", and Engelder, T.E.W., Popular Symbolics. St. Louis: Concordia Publishing House, 1934. pp. 135-6, Part XXXIX. "Eternal Death", paragraph 196.
- Luke 23:42-43, 2 Cor. 5:8, Engelder, T.E.W., Popular Symbolics. St. Louis: Concordia Publishing House, 1934. pp. 130, Part XXXIV. "The State of the Soul in the Interval Between Death and the Resurrection", paragraph 185.
- 1 Cor. 15:22–24, Francis Pieper, Christian Dogmatics, 505-515; Heinrich Schmid, The Doctrinal Theology of the Evangelical Lutheran Church, 624-632; John Mueller, Christian Dogmatics, 616-619
- Beeke, Joel (2011). "The Assurance Debate". In Haykin, Michael A.G.; Jones, Mark (eds.). Drawn Into Controversie: Reformed Theological Diversity and Debates Within Seventeenth-Century British Puritanism. Göttingen, Germany: Vandenhoeck & Ruprecht. p. 273.
- Compare the Westminster Confession of Faith, Chapter XVIII.
- Westminster Confession of Faith, Chapter 18, paragraph 3
- "What's the difference between New Order and Old Order Amish?". Amish America. 2010. Retrieved 22 May 2021.
- Fretz, Clarence Y. "How To Make SURE You Are Saved". Anabaptists. Retrieved 22 May 2021.
- The text of the Council of Trent translated by J. Waterworth, 1848.
- Part 20 of 25 of the Justification by Faith debate with Protestant apologist James White.
- Ludwig Ott, Fundamentals of Catholic Dogma IV/I § 22.
- Ludwig Ott, Fundamentals of Catholic Dogma IV/I § 12.
- St. Thomas Aquinas, S. th. II/II 18 IV
- see S. th. II/II 21 I for the differences. - Let it be said, for the sake of those readers that may feel difficulty in overcoming some sins whatever, that hope in even only a future repenting, combined with a certain sorrow for the sins, is as such valid hope; that not hoping is never a legitimate way of avoiding presumption; and that despair is worse than presumption, as St. Thomas teaches in S. th. II/II 21 II.
- See Sollier, Joseph. "Final Perseverance." The Catholic Encyclopedia. Vol. 11. New York: Robert Appleton Company, 1911. 6 Nov. 2011 < http://www.newadvent.org/cathen/11711a.htm>.
- Lochman, Jan Milič (1999), "Assurance of Salvation", in Fahlbusch, Erwin (ed.), Encyclopedia of Christianity, 1, Grand Rapids: Wm. B. Eerdmans, pp. 146–147, ISBN 0802824137
- Questions of Clarification for Wesley's Doctrine of Assurance by Michael E. Lodahl
- The Relationship of Assurance to Justification and Regeneration in the Thought of John Wesley by Scott Kisker
- That We Know Him: The Doctrine of Assurance by Michael Avery (Wesleyan-Arminian)
- Sermon #10: "The Witness of the Spirit, Part 1" by John Wesley
- Sermon #11: "The Witness of the Spirit, Part 2" by John Wesley
- Sermon #12: "The Witness of Our Own Spirit" by John Wesley
- Heaven on Earth: a Treatise on Christian Assurance by Brooks, Thomas, Edinburgh: The Banner of Truth Trust, 1961. First published 1654 ISBN 0-85151-356-5
- Guthrie, William (1620–1665). "The Christian's Great Interest". Retrieved 2010-07-23.
- Ryle, J.C.. "Assurance". Retrieved 2010-07-23.
- Assemblies of God position on Assurance of Salvation
- Monergism.com: Directory of Theology: Assurance