We must not think that Christian faith is a pure and simple knowledge of God, or an understanding of the Scripture, which flutters about in the brain without touching the heart. That is the opinion we normally hold of things which are validated for us by some reason which sounds probable.
Christian faith is, rather, a firm and solid assurance of the heart, by which we cling securely to the mercy of God which is promised to us through the gospel.
Thus the definition of faith must be taken from what underlies the promise. And faith is so very much built on this foundation that it would immediately collapse, or, rather, completely vanish, if this foundation were taken away.
Hence when the Lord presents to us His mercy through the promise of the gospel, if we entrust ourselves to Him who made the promise, and if we do this with certainty and without any hesitation, it is then that we lay hold of His Word by faith.
And this definition is no different from that of the apostle, who teaches us that faith is the reality of the things we hope for, the expression of what we do not see (Hebrews 11:1). By that, the apostle means a sure and certain possession of the things God promises, and a manifestation of things that are not physically visible — that is to say, the eternal life which we hope to have by reason of our trust in this divine generosity which is given to us through the gospel.
Now since all God’s promises are confirmed in Christ and, so to speak, kept and accomplished in Him, it is clear that Christ is indisputably the perpetual object of faith. And it is in that object that faith contemplates all the riches of divine mercy.
John Calvin, Truth for All Time