Confirming (epistērizontes). Late verb (in lxx), in N.T. only in Act 14:22; Act 15:32, Act 15:41, to make more firm, to give additional (epi) strength. Each time in Acts the word is used concerning these churches.
To continue in the faith (emmenein tēi pistei). To remain in with locative, old verb. It is possible that pistis here has the notion of creed as Paul uses it later (Col 1:23 with epimenō; 1Ti 5:8). It seems to be here more than trust or belief. These recent converts from heathenism were ill-informed, were persecuted, had broken family and social ties, greatly needed encouragement if they were to hold out.
We must (dei hēmās). It does not follow from this use of “we” that Luke was present, since it is a general proposition applying to all Christians at all times (2Ti 3:12). Luke, of course, approved this principle. Knowling asks why Timothy may not have told Luke about Paul’s work. It all sounds like quotation of Paul’s very language. Note the change of construction here after parakalountes (infinitive of indirect command, emmenein, but hoti dei, indirect assertion). They needed the right understanding of persecution as we all do. Paul frankly warned these new converts in this heathen environment of the many tribulations through which they must enter the Kingdom of God (the culmination at last) as he did at Ephesus (Act 20:20) and as Jesus had done (Joh 16:33). These saints were already converted.