Also, there are very many who so conceive of God’s mercy that they receive almost no consolation from it. They are constrained with miserable anxiety at the same time as they are in doubt whether he will be merciful to them because they confine that very kindness of which they seem utterly persuaded within too narrow limits. For among themselves they ponder that it is indeed great and abundant, shed upon many, available and ready for all; but that it is uncertain whether it will even come to them, or rather, whether they will come to it….
Therefore, it does not so much strengthen the spirit in secure tranquility as trouble it with uneasy doubting. But there is a far different feeling of full assurance that in the Scriptures is always attributed to faith…there is no right faith except when we dare with tranquil hearts to stand in God’s sight. – Calvin, Institutes 3.2.15.
Here indeed is the chief hinge on which faith turns: that we do not regard the promises of mercy that God offers as true only outside ourselves, but not at all in us; rather that we make them ours by inwardly embracing them. Hence, at last is born that confidence which Paul elsewhere calls “peace” [Rom. 5:1],… Now it is an assurance that renders the conscience calm and peaceful before God’s judgment. Without it the conscience must be harried by disturbed alarm, and almost torn to pieces; unless perhaps, forgetting God and self, for the moment sleeps. And truly for the moment, for it does not long enjoy that miserable forgetfulness without the memory of divine judgment repeatedly coming back and very violently rending it…. the apostle does not consider the eyes of our minds well illumined, except as we discern what the hope of the eternal inheritance is to which we have been called [Eph. 1:18]. And everywhere he so teaches as to intimate that we cannot otherwise well comprehend the goodness of God unless we gather from it the fruit of great assurance. – Institutes, 3.2.16
Still, someone will say: ‘Believers experience something far different: In recognizing the grace of God toward themselves they are not only tried by disquiet, which often comes upon them, but they are repeatedly shaken by gravest terrors. For so violent are the temptations that trouble their minds as not to seem quite compatible with that certainty of faith.’ Accordingly,we shall have to solve this difficulty if we wish the above-stated doctrine to stand. Surely, while we teach that faith ought to be certain and assured, we cannot imagine any certainty that is not tinged with doubt, or any assurance that is not assailed by some anxiety. On the other hand, we say that believers are in perpetual conflict with their own unbelief. Far, indeed, are we from putting their consciences in any peaceful repose, undisturbed by any tumult at all. Yet, once again, we deny that, in whatever way they are afflicted, they fall away and depart from the certain assurance received from God’s mercy. – Institutes 3.2.17