From First and Second Timothy, Titus by George T. Montague, commenting on 2 Timothy 4:6-8, 16-18
What exactly does Paul mean in saying I have kept the faith? The word “faith” (pistis) has so many different meanings in the Bible and even in Paul that it is not easy to decide which meaning is intended. The parallelism with the preceding two phrases and the overall tone of the Pastorals might suggest that he has kept the deposit of the faith and handed it on faithfully (2 Tim 2:2), hence “the faith” in its objective sense. This is what he has urged Timothy to do (1 Tim 6:20); thus Paul would again be pointing to his own example.
However, Timothy surely knows that. Hence it may be preferable to understand the phrase in the sense “I have kept my pledge to the end,” “I have been faithful to my baptismal commitment,” or some equivalent. This would correspond to the widespread use of pistis in the Hellenistic world for fidelity to an oath or for trustworthiness. Josephus uses the word for marital fidelity. It was also the supreme virtue of the soldier, bound by sacred oath (sacramentum) to the emperor. So highly was it held in esteem in Rome that a temple was erected to the goddess Faith (Pistis): Numa “was the first to build temples to Faith . . . and he taught the Romans their most solemn oath by Faith, which they still continue to use.”
Of the three marks of triumph, Paul puts fidelity as the climax. In the Greek there is a beautiful rhetorical flourish in the similar sounding verbs teteleka (“I have finished”) and tetereka (“I have kept”). As Jesus having “loved his own in the world . . . loved them to the end” (John 13:1), so Paul has been faithful to the end.