Properly speaking, or perhaps we should say, speaking psychologically or mentally, faith seems to be an attribute of the intellect, rather than of the heart; an act or state of the understanding rather than of the sensibilities. And yet it must be admitted, that, in the order of mental sequence, it is a state of mind, which, in consequence of being subsequent to perceptions, lays nearer the heart, is in much closer proximity with it, than some other intellectual states or acts. But this is not the only or the most important particular to be considered here. The important fact, and the only one which can give a satisfactory explanation of what is denominated the faith of the heart, is the law of mental relation and action just now stated, viz.: that religious affection is consequent on religious faith, and that they correspond to each other in degree. A faith of the heart, then, is a faith, which affects the heart. A faith of the heart is a faith, which works by love. “In Jesus Christ,” says the Apostle, “neither circumcision availeth any thing nor uncircumcision, but faith which worketh by love.” Galatians 5:6.
I suppose, that there may be, and that there probably is a sort of faith, either so general and unspecific in its nature, or so weak in its degree, that it does not produce love. A man, for instance, may believe in Jesus Christ as a mere man, as an inhabitant of Judea in the time of Pontius Pilate, and as a very remarkable and good man. But this belief, which does not seem to differ from that which we have in Confucius and Socrates, never is, and never can be the source of such feelings, as those which naturally follow our belief in Christ as one sent from God, as the beloved son of the Father, as an authorized teacher, and as an atoning sacrifice. And then, again, our faith, even if it be right in other respects, may be so weak, so vacillating, so closely allied to actual skepticism, as to fail of being followed by that love, which purifies the heart; the only love which can be acceptable to God. The faith of the heart, therefore, is that faith, which makes a new heart; in other words, which inspires new affections; such affections, as are conformable to God’s law and will.
—adapted from The Life of Faith (1852) Part 1, Chapter 6 (emphasis added).