The readers (hearers) can call upon their own knowledge of Paul’s time among them to confirm what he is about to say, and if there are matters beyond their knowledge or understanding (such as the hidden motives of Paul and his coworkers), God himself can bear testimony to those (see 1 Thessalonians 2:5).
10 You are witnesses, and God also, how holy and righteous and blameless was our conduct toward you believers.