”We have also seen, that since the knowledge of the divine goodness cannot be of much importance unless it leads us to confide in it, we must exclude a knowledge mingled with doubt, – a knowledge which, so far from being firm, is continually wavering. But the human mind, when blinded and darkened, is very far from being able to rise to a proper knowledge of the divine will; nor can the heart, fluctuating with perpetual doubt, rest secure in such knowledge. Hence, in order that the word of God may gain full credit, the mind must be enlightened, and the heart confirmed, from some other quarter. We shall now have a full definition of faith, if we say that it is a firm and sure knowledge of the divine favor toward us, founded on the truth of a free promise in Christ, and revealed to our minds, and sealed on our hearts, by the Holy Spirit.” (emphasis mine)

From the Institutes Book 3.2.7

Ref: http://www.reformed.org/master/index.html?mainframe=/books/institutes/books/indxbk3.html