But as for you translates an expression that is identical to that found in the beginning of verse 10. The sense of But is contrastive; the false teachers are “progressing” in their wickedness, but as for Timothy, he should advance in godliness by continuing in the truth that he has learned.

Continue is literally “remain,” “keep on,” “do not waver.” (See further on “continues” in 1 Tim 2.15.) Firmly believed translates a verb that means “to come to believe something as true,” “to be convinced and certain of the truth of something.” Both what you have learned (literally “what you were taught”) and have firmly believed are in the aorist tense, focusing on the times and occasions in the past when Timothy learned these truths and came to believe in them. But while the tense is aorist, it makes more sense, in English at least, to translate the aorist as perfect, which is what Revised Standard Version has done. Good News Translation retains the aorist in the first case but interprets the second aorist as having a stative sense; hence “firmly believe.”

What does it mean to continue in what you have learned? It can be “continue to live according to,” “continue to be loyal to” (compare Contemporary English Version “Keep on being faithful,” New American Bible, Revised “remain faithful,” Translator’s New Testament “you must be loyal”), “continue to believe,” “continue to make progress,” “hold on to,” “stand firm in” (compare Revised English Bible “stand by,” New Jerusalem Bible “you must keep to”).

Timothy should hold on to these truths because he knows that his teachers are reliable Christians and can therefore be trusted. The pronoun whom (in from whom you learned it) is plural, indicating that Paul was not only talking of himself but of others as well who played a part in the Christian education of Timothy, including Timothy’s own mother and grandmother, who are mentioned in 1.5.

An alternative translation model for this verse is:
• As for you, Timothy, you must continue to hold on faithfully to the things that people taught you and that you believe in. After all, you know who taught you these truths (or, doctrines).

Quoted with permission from Arichea, Daniel C. and Hatton, Howard A. A Handbook on Paul’s Second Letter to Timothy. (UBS Handbook Series). New York: UBS, 1995. For this and other handbooks for translators see here .