For now we live, if ye stand fast in the Lord.
Now - as the case is; seeing ye stand fast.
We live - we flourish. It revives us in affliction to hear of your stedfastness (Psalms 22:26; 3 John 1:3-4).
If. The vivid joy which Paul "now" feels will continue, if the Thessalonians continue stedfast. They still needed exhortation (1 Thessalonians 3:10); therefore he subjoins, "if ye," etc. (Philippians 4:1.)