John Calvin: "Now we shall possess a right definition of faith if we call it a firm and certain knowledge of God’s benevolence toward us, founded upon the truth of the freely given promise in Christ, both revealed to our minds and sealed upon our hearts through the Holy Spirit" (Institutes 3.2.7).
John Calvin: "[Paul] teaches us that our confidence in this respect is made certain by the Spirit of adoption, who could not inspire us with confidence in prayer without sealing to us the gratuitous pardon" (Comm. on Romans 8:15).
John Calvin: "Lastly, there was another most pestilential error, which not only occupied the minds of men, but was regarded as one of the principal articles of faith, of which it was impious to doubt: that is, that believers ought to be perpetually in suspense and uncertainty as to their interest in the divine favor. By this suggestion of the devil, the power of faith was completely extinguished, the benefits of Christ’s purchase destroyed, and the salvation of men overthrown. For, as Paul declares, that faith only is Christian faith which inspires our hearts with confidence, and emboldens us to appear in the presence of God (Rom. 5:2). On no other view could his doctrine in another passage be maintained: that is, that 'we have received the Spirit of adoption, whereby we cry, Abba, Father' (Rom. 8:15)" (The Necessity of Reforming the Church, p. 27).
Sinclair B. Ferguson: "Cardinal Robert Bellarmine [1542-1621], perhaps the most formidable Roman Catholic theologian of the sixteenth century, gave striking expression to this when he claimed that assurance is the greatest of all Protestant heresies ... Judging by the emphasis Calvin would after place (in various contexts) on certainty in the Christian life, it seems likely that coming to an assured knowledge of God and the forgiveness of sins in Christ was a major element in his conversion" ("Calvin the Man: A Heart Aflame" in Joel R. Beeke and Garry J. Williams (eds.), Calvin, Theologian and Reformer [Grand Rapids, MI: Reformation Heritage Books, 2010], pp. 12-13).
Anthony N. S. Lane: "There are strands of the Reformed, Calvinist tradition for which assurance has become a problem. This is especially acute among some circles that claim assurance of salvation is almost seen as presumptuous. An illustration is used that a sheep has a mark of ownership on its ear that can be seen by all—except by the sheep itself. The message is clear. If you are a Christian, it should be plain to everyone—except yourself. In those circles, there is a tradition of people noted for their great sanctity refraining from actually claiming to be converted. Indeed reluctance to claim this is itself at times seen as evidence of sanctification. Allied to this is the myth that Calvin denied that we can know whether we are elect and that he himself died in despair. Both of these are totally untrue. There is no shortage of evidence about his last days, and he clearly died confident of salvation. Again, so far was he from teaching that it is impossible to have assurance of salvation that he actually held that assurance of salvation is itself part of saving faith. (In doing so, he was following in the steps of Luther, Philip Melanchthon, and other mainstream Reformers.) This followed from his definition of faith, already quoted: "a firm and certain knowledge of God's benevolence toward us, founded upon the truth of the freely given promise in Christ, both revealed to our minds and sealed upon our hearts through the Holy Spirit" ("Calvin's Way of Doing Theology: Exploring the Institutes" in Joel R. Beeke and Garry J. Williams (eds.), Calvin, Theologian and Reformer [Grand Rapids, MI: Reformation Heritage Books, 2010], pp. 50-51).
Sinclair B. Ferguson: "It is therefore something of a paradox that in some strands of the Reformed tradition believers have been discouraged from enjoying any assurance of their sonship. What good father in this world would want to bring his children up without the assurance that they are his children? Would the Father of lights (James 1:17) do that? The model for all true fatherhood is rooted in the fatherhood of God. Calvin considers this truth to be a glorious liberation, in some senses his own parallel to Luther's appreciation of justification. The God of all glory not only becomes our Father, but wishes to assure His children that this is so. That is why Calvin says in Institutes 3.2.7 that we possess a right definition of faith only when we think of it as "a firm and certain knowledge of God's benevolence towards us" ("Calvin and Christian Experience: The Holy Spirit in the Life of the Christian" in Joel R. Beeke and Garry J. Williams (eds.), Calvin, Theologian and Reformer [Grand Rapids, MI: Reformation Heritage Books, 2010], pp. 102-103).
II. Heidelberg Catechism and Zacharias Ursinus
Heidelberg Catechism, Q. & A. 1: "What is thy only comfort in life and death? That I with body and soul, both in life and death, am not my own, but belong unto my faithful Saviour Jesus Christ; who, with His precious blood, hath fully satisfied for all my sins, and delivered me from all the power of the devil; and so preserves me that without the will of my heavenly Father, not a hair can fall from my head; yea, that all things must be subservient to my salvation, and therefore, by His Holy Spirit, He also assures me of eternal life, and makes me sincerely willing and ready, henceforth, to live unto Him."
Heidelberg Catechism, Q. & A. 21: "What is true faith? True faith is not only a certain knowledge, whereby I hold for truth all that God has revealed to us in His Word, but also an assured confidence, which the Holy Ghost works by the gospel, in my heart; that not only to others, but to me also, remission of sin, everlasting righteousness and salvation, are freely given by God, merely of grace, only for the sake of Christ's merits."
Heidelberg Catechism, Q. & A. 53: "What dost thou believe concerning the Holy Ghost? First, that He is true and co-eternal God with the Father and the Son; secondly, that He is also given me, to make me by a true faith, partaker of Christ and all His benefits, that He may comfort me and abide with me for ever."
Heidelberg Catechism, Q. & A. 54: "What believest thou concerning the "holy catholic church" of Christ? That the Son of God from the beginning to the end of the world, gathers, defends, and preserves to Himself by His Spirit and Word, out of the whole human race, a church chosen to everlasting life, agreeing in true faith; and that I am and for ever shall remain, a living member thereof."
Zacharias Ursinus: "If you mean that we cannot say with certainty that one will be saved, you are right when speaking of others; but with regard to oneself, or one’s own conscience and convictions concerning oneself, such a conception is both shocking and blasphemous, and subverts the very foundation of faith. Whoever has taught you such an idea, has instructed you as would a devil, even though he came from heaven. I will say even more; if you are not certain in this world that you are an heir of eternal life, you will not be one after death. From such a fate the Lord deliver you. For faith itself is that certainty which is the beginning of eternal life, which beginning everyone must possess in this life who would have it hereafter. If you would remember the meaning of the word hope, that it is a certain expectation of eternal life, you would not write to me what causes my hair to stand on end. I would not accept a hundred thousand worlds and be so far away from my Lord as not to know certainly whether I am His or not" (in a private letter, quoted in Otto Thelemann, An Aid to the Heidelberg Catechism, pp. 452-453; and in Fred Klooster, Our Only Comfort, vol. 1, p. 228).
III. Others
Octavius Winslow: "Faith brings assurance, and assurance is faith. The measure of our assured interest in Christ, will be the measure of our faith in Christ. This is the true definition of assurance, the nature of which is a question of much perplexity to sincere Christians. Assurance is not something audible, tangible, or visionary—a revelation to the mind, or a voice in the air. Assurance is believing. Faith is the cause, assurance is the effect. Assurance of personal salvation springs from looking to, and dealing only with Jesus. It comes not from believing that I am saved, but from believing that Christ is my Savior ... One simple, believing sight of Christ will produce more light and peace and joy than a lifetime of looking within ourselves for evidences and signs of grace. All the sinner's merit, all his worthiness, beauty, and salvation, is centered in Christ, is from Christ, and Christ alone. And, by simply believing the great truth of the gospel, that Christ died for sinners—receiving Christ, not purchasing Him,—as God's 'unspeakable gift' of free and sovereign grace, will awaken in the soul the assured and grateful acknowledgment, 'He loved me, and gave Himself for me'" (Soul Heights and Soul Depths, pp. 2-3).
Frederick Dale Bruner: "For Paul, the ability to cry 'Father!' was the work and therefore the evidence of the Spirit of the Son. The evidence of the Spirit is first of all Christian faith in God the Father or—Christian prayer. The Spirit in these parallel texts is deliberately called 'the Spirit of (the) Son(ship),' not only because the Spirit belongs to the Son and is given in him, but because it is the Spirit's work to assure believers that they are, through the Son, truly sons of God. The gift of the Spirit is first of all the subjective assurance of the gift of adoption or justification. The 'objective' justification and the 'subjective' gift of the Spirit cannot be separated for one is acceptance with God and the other is knowledge of this acceptance. And what God has joined together no man should put asunder. The 'objective' justification never remains merely objective but is always accompanied mediately in the Word by its 'subjective' revelation, and this is the first work of the Spirit—this is the Spirit. His evidence is Christian assurance." (A Theology of the Holy Spirit: The Pentecostal Experience and the New Testament Witness [Grand Rapids, MI: Eerdmans, 1970], pp. 268-269).
Robert Letham: "Christian assurance suffered. Since the promises of God were made only to those who had met the stipulated conditions, a laborious process of self-examination and introspection was needed if one was to be sure of salvation. Exponents of this view, which, although never unanimously held, gained ground from the end of the sixteenth century, lost sight of the crucial point that the covenant of grace centred upon and was fulfilled by Christ on our behalf" (The Work of Christ [Leicester: IVP, 1993], p. 52).