In his excellent teaching on the meaning of faith, J.I. Packer tackles the age-old question of how justification by faith alone results in anything other than spiritual sloth and antinomianism. He writes the following:
Faith abandons hope in man’s own accomplishments, leaves all works behind, and comes to Christ alone and empty-handed, to cast itself on mercy. Such is the faith that saves.
But does this mean that saving faith throws a halo over idleness, and that the gospel of justification by faith only is really hostile to moral endeavour? Indeed not. ‘Faith is a lively thing,’ wrote Luther, ‘mighty in working, valiant and strong, ever doing, ever fruitful; so that it is impossible that he who is endued therewith should not work always good works without ceasing … for such is his nature.’
What saves is faith alone, but the faith that saves is never alone; it is always ‘working through love (Gal. 5:6), becoming a moral dynamic of unparalleled power in the believer’s life. The proof that a man’s faith is real is precisely this — that it makes him work. How does it do this? By making him feel the constraint of Christ’s love for him, and the greatness of the debt of gratitude which he owes to his God. As we said once before, Christian doctrine is grace, and Christian conduct is gratitude. The believer does not do what he does as a means to being justified, but there are no limits to what he will do for his Lord out of gratitude for the justification that he has received.