
True faith is both an infallible assurance, and a particular assurance of the remission of sins, and of life everlasting. And therefore by this faith, a man may be certainly and particularly assured of the remission of sins, and life everlasting...
Paul says of Abraham that he did not doubt the promise of God through unbelief, but was strengthened in faith, and gave glory to God being fully assured, that he which promised was able to do it, where I observe first that doubting is made a fruit of unbelief. Therefore, infallible certainty and assurance, being contrary to doubting, must proceed from true faith, considering that contrary effects come of contrary causes, and contrary causes produce contrary effects.
Second, I note, that the strength of Abraham's faith, did stand in fullness of assurance; for the text says, he was strengthened in the faith, being fully assured. And again, Heb 11:1, true saving faith is said to be the ground and substance of things hoped for, and the evidence or demonstration of thing that are not seen. But faith can be no ground or evidence of things, unless it be for nature certainty itself...
That saving faith is a particular assurance is proved by this, that the property of faith is to apprehend and apply the promise, and the thing promised, Christ with his benefits...In these words, to believe in Christ, and to receive Christ, are put for one and the same thing. Now to receive Christ, is to apprehend and apply him with all his benefits unto ourselves, as he is offered in the promises of the gospel. William Perkins, The Workes: A Reformed Catholike, Vol. I (John Legatt: London, 1616) 564.