John Wesley taught (e.g. in the two sermons on 'The Witness of the Spirit') that, according to the NT, it is the privilege of ordinary Christian believers to experience 'the witness of God's Spirit with their spirit that they are children of God' (Rom. 8:16). He claimed that 'this great evangelical truth has been recovered [by the Methodists] which has been for many years well-nigh lost and forgotten.'
Assurance has been regarded ever since as one of the key doctrinal emphases of Methodism. Wesley regarded it not only as fully scriptural, but also as confirmed by his own experience at Aldersgate Street and by that of many of his followers. The doctrine was criticized by Bishop Joseph Butler and others, who regarded it as conducing to presumptuousness, enthusiasm and perhaps spiritual pride, and as creating too great a dependence on 'feeling' in the Christian life; hence E.B. Pusey's later charge that Methodists preached 'justification by feeling'.
Wesley hedged the doctrine with safeguards. He did not regard the experience as necessary to salvation. Moreover, he recognized that it was all too possible to fall from grace, even after an experience of present salvation. In the second of his two sermons on the doctrine (1767) he acknowledged that Christians might have periods of doubt in which the sense of assurance (though not the relationship with God in which it was grounded) was lost. The doctrine may be seen as the logical corollary of belief in 'adoption' as the means of our incorporation into Christ and in the restoration of the divine image in us.