"we know that God chose you to become his people" (UDB), or "we know that God chose you to serve him in a special way."
## we know ##
The word "we" refers to Paul, Silvanus and Timothy but not the Thessalonian believers. (See [[rc://en/ta/man/translate/figs-exclusive]])
## but also in power, in the Holy Spirit ##
Possible meanings are 1) "Paul and his companions preach powerfully by being enabled by the Holy Spirit" or 2) "The Gospel has a powerful effect on the believers through the conviction work of the Holy Spirit."