In Summa theologiae II-II, q. 4, a. 4, Saint Thomas Aquinas examines James 2:24 and the faith that does not justify. Thomas distinguishes between “faith formed by love” and “faith not formed by love”. Thomas says that the faith of each is one and the same. They are not two different kinds of faith. Rather, love must form the faith. Otherwise, the faith is not salvific. Protestants don’t often think of faith in these terms and this leads to confusion about the Catholic position on faith and justification.

Thomas’ reply to objection 3 is especially illuminating:

Grace causes faith not only when faith begins anew to be in a man, but also as long as faith lasts. For it has been said above (I, 104, 1; I-II, 109, 9) that God is always working man’s justification, even as the sun is always lighting up the air. Hence grace is not less effective when it comes to a believer than when it comes to an unbeliever: since it causes faith in both, in the former by confirming and perfecting it, in the latter by creating it anew.

This conclusion entails that a man is not justified by faith alone, because a man might have faith but his faith may not be formed by love, i.e. a man believes the entire Christian faith and believes in Christ, but his heart is cold toward God and toward his neighbor.