The words translated holy and righteous and blameless are actually adverbs in Greek. By using adverbs, Paul seems to be placing the emphasis on behaviour and conduct more than on character;1,2 but it is impossible to translate this into English without some sort of paraphrase. Thus the ESV translates how holy and righteous and blameless was our conduct toward you believers
(adding in the words our conduct
). The NASB, which retains the adverbs, must change the verb from were
to behaved
to come up with the translation how devoutly and uprightly and blamelessly we behaved toward you believers.
There is not much to choose between the different translations, but it should be understood that Paul is drawing attention to his conduct while in Thessalonica.
10 You are witnesses, and God also, how holy and righteous and blameless was our conduct toward you believers.