Now as Jannes and Jambres withstood Moses, so do these also resist the truth: men of corrupt minds, reprobate concerning the faith. Now as Jannes and Jambres withstood Moses - This refers to the history of the Egyptian magicians, given in Exodus 7 (see on Exodus 7:11 (note) and Exodus 7:12 (note)), and particularly the concluding observations at the end of that chapter, (note) where several things are said concerning these two men.
Men of corrupt minds - It appears as if the apostle were referring still to some Judaizing teachers who were perverting the Church with their doctrines, and loudly calling in question the authority and doctrine of the apostle.
Reprobate concerning the faith - Αδοκιμοι· Undiscerning or untried; they are base metal, unstamped; and should not pass current, because not standard. This metaphor is frequent in the sacred writings.