Jamieson, Fausset and Brown on 2 Timothy 3:8

8. Now--Greek, "But"; it is no wonder there should be now such opponents to the truth, for their prototypes existed in ancient times [Alford].

Jannes ... Jambres--traditional names of the Egyptian magicians who resisted Moses (Ex 7:11, 22), derived from "the unwritten teaching of the Jews" [Theodoret]. In a point so immaterial as the names, where Scripture had not recorded them, Paul takes the names which general opinion had assigned the magicians. Eusebius [Preparation of the Gospel], quotes from Numenius, "Jannes and Jambres were sacred scribes (a lower order of priests in Egypt) skilled in magic." Hiller interprets "Jannes" from the Abyssinian language a trickster, and "Jambres" a juggler" (Ac 13:8).

resist--"withstand," as before. They did so by trying to rival Moses' miracles. So the false teachers shall exhibit lying wonders in the last days (Mt 24:24; 2Th 2:9; Re 13:14, 15).

reprobate--incapable of testing the truth (Ro 1:28) [Bengel]. Alford takes passively, "not abiding the test"; rejected on being tested (Jr 6:30).