William Cunningham on the Reformers and the doctrine of assurance

[Reformed Covenanter]

... The Reformers, in order to show that the assurance which might be attained without either a special revelation or the testimony of the church, was full and perfect, were led to identify it with our belief in the doctrines of God’s Word, and to represent it as necessarily included or implied in the act or exercise of justifying and saving faith; nay, even sometimes to give it as the very definition of saving faith, that it is a belief that our own sins have been forgiven, and that we have been brought into a state of grace. This seemed to be an obvious and ready method of giving to the belief of our personal safety for eternity the very highest degree of certainty, and hence many of the Reformers were tempted to adopt it.

This view was certainly exaggerated and erroneous. It is very evident that no man can be legitimately assured of his own salvation simply by understanding and believing what is contained or implied in the actual statements of Scripture. Some additional element of a different kind must be brought in, in order to warrant such an assurance; something in the state or condition of the man himself must be in some way ascertained and known in order to this result. It may not, indeed, always require any lengthened or elaborate process of self-examination to ascertain what is needful to be known about men themselves, in order to their being assured that they have been brought into a state of grace. ...

For more, see William Cunningham on the Reformers and the doctrine of assurance.