Speaking on the Heidelberg Catechism Q&A 91, the author Zacharias Urisnus comments on the necessity of good works,
"It is by our good works, therefore, that we know that we possess true faith, because the effect is not without its own proper cause, which is always known by its effect; so that if we are destitute of good works and new obedience, we are hypocrites, and have an evil conscience instead of true faith; for true faith (which is never wanting in all the fruits which are peculiar to it,) as a fruitful tree produces good works, obedience and repentance; which fruits distinguish true faith from that faith which is merely historical and temporary, as well as from hypocrisy itself." — Zacharias Ursinus, Commentary on the Heidelberg Catechism
h/t David Carnes