We are at this point such enemies of our own salvation, because of our natural corruption , that if God had merely contented Himself to tell us that we shall find our salvation in Jesus Christ, we would only mock it; thus has the world always done and will do until the end . Even more, if He added nothing more than to tell us also that the means whereby we experience the efficacy of this remedy against eternal death is to believe in Jesus Christ, that would profit us nothing. For, in all this, we are more than dumb, deaf, and blind through the corruption of our nature. It would be no more possible for us even to wish to believe than it would be for a dead man to fly. It is necessary therefore that with all this, the good Father, who chose us for His glory, should come to multiply His mercy towards His enemies. In declaring to us that He has given His own only Son so that whosoever takes hold of Him by faith should not perish, He creates also in us this instrument of faith which He requires from us.
Now, the faith of which we speak does not consist only in believing that God is God, and that the contents of His Word are true:- for the devils indeed have this faith, and it only makes them tremble -- But we call 'faith' a certain knowledge which, by His grace and goodness alone, the Holy Spirit engraves more and more in the hearts of the elect of God. By this knowledge, each of them, being assured in his heart of his election, appropriates to himself and applies to himself the promise of his salvation in Jesus Christ.