These principles help us to understand what is meant by the faith of the heart; a form of expression which we frequently hear. 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.
— from The Life of Faith (1852) Part 1, Chapter 6.