And faith has power to do this. Faith can make a new heart; and nothing but faith can do it. In saying this, it will be naturally understood, that we speak of the mind and of mental sequence; in other words, of that which takes place in the mind and in the mental order, and not of any thing which takes place out of it and above it. We speak of secondary relations and agency; and not of him, who, in being the primary agent, is the life of the mind itself. We say, therefore, that, in the order of mental succession, and in the gradation of mental influence, faith stands first; first in time, and first in power; and that, in this view of the subject, we may properly speak of faith as having a creative agency, and as making a new heart. If faith be imperfect in degree, it will of course be followed by imperfect issues; it will make a heart imperfect as itself. But if it be strong, if it be assured, it will give a strong, an assured heart. If it be Abraham’s faith, it will give Abraham’s heart. If it be Paul’s faith, it will give Paul’s heart. If it be the faith which Christ had, a faith, which Satan’s arts could not shake, and man’s hostility could not perplex, and even the hiding of his Father’s countenance, could not discourage, we cannot hesitate to say with reverential gratitude, that it will give Christ’s consecrated heart; a heart which never falters in the cause of truth and duty; a heart that can be nailed to the Cross for God’s name and God’s glory.
And this takes place, as we have already intimated, not accidentally, but by an immutable law. Eternal law is at the bottom; and, therefore, eternal truth is in it. It is the law of men, the law of angels; and we might add, with the simple modification that what is faith in the human mind becomes knowledge in the divine mind, that it is the law of God. God loves, and he can love, only what he knows to be a proper object of love. In men, who are not the subjects of absolute knowledge, faith takes the place of such knowledge; and they love, and can love, only as they believe. “Believe,” says Archbishop Leighton, “and you shall love. Believe much, and you shall love much.” And carrying out the principle to its legitimate issues, I think we may add with safety, Believe with all your powers of belief, and you will love with all your powers of love. Believe with assurance of faith, and you will love with assurance of love. In other words, believe perfectly, and you will love perfectly.
— from The Life of Faith (1852) Part 1, Chapter 6.