To "stand firm" is an idiom meaning to continue being faithful. AT: "if you continue to trust in the Lord" (See: [[rc://en/ta/man/translate/figs-idiom]])
This rhetorical question can be expressed as a statement. AT: "We cannot thank God enough for what he has done for you! We greatly rejoice over you when we pray to our God!" (See: [[rc://en/ta/man/translate/figs-rquestion]])
Paul speaks as if he and his companions were physically in God's presence. He is probably referring to the activity of praying. (See: [[rc://en/ta/man/translate/figs-metaphor]])