en_tn_lite_do_not_use/1th/01/05.md

not in word only

"not only in what we said"

but also in power, in the Holy Spirit

Possible meanings are 1) the Holy Spirit gave Paul and his companions the ability to preach the gospel powerfully or 2) the Holy Spirit made the preaching of the gospel have a powerful effect among the Thessalonian believers or 3) the Holy Spirit demonstrated the truth of the gospel preaching by means of miracles, signs, and wonders.

power, in the Holy Spirit, and in much assurance

The abstract noun "assurance" can be translated as a verb. Alternate translation: "power and in the Holy Spirit, and God made you sure that it was true" (See: rc://en/ta/man/jit/figs-abstractnouns)

what kind of men

"how we conducted ourselves when"