18. "But some one will say": so the Greek. This verse continues the argument from Jas 2:14, 16. One may say he has faith though he have not works. Suppose one were to say to a naked brother, "Be warmed," without giving him needful clothing. "But someone (entertaining views of the need of faith having works joined to it) will say (in opposition to the 'say' of the professor)."
      show me thy faith without thy works--if thou canst; but thou canst not SHOW, that is, manifest or evidence thy alleged (Jas 2:14, "say") faith without works. "Show" does not mean here to prove to me, but exhibit to me. Faith is unseen save by God. To show faith to man, works in some form or other are needed: we are justified judicially by God (Ro 8:33); meritoriously, by Christ (Isa 53:11); mediately, by faith (Ro 5:1); evidentially, by works. The question here is not as to the ground on which believers are justified, but about the demonstration of their faith: so in the case of Abraham. In Ge 22:1 it is written, God did tempt Abraham, that is, put to the test of demonstration the reality of his faith, not for the satisfaction of God, who already knew it well, but to demonstrate it before men. The offering of Isaac at that time, quoted here, Jas 2:21, formed no part of the ground of his justification, for he was justified previously on his simply believing in the promise of spiritual heirs, that is, believers, numerous as the stars. He was then justified: that justification was showed or manifested by his offering Isaac forty years after. That work of faith demonstrated, but did not contribute to his justification. The tree shows its life by its fruits, but it was alive before either fruits or even leaves appeared.

JFB.