Jamieson, Fausset & Brown

now--as the case is; seeing ye stand fast. we live--we flourish. It revives us in our affliction to hear of your steadfastness (Psa 22:26). if--implying that the vivid joy which the missionaries "now" feel, will continue if the Thessalonians continue steadfast. They still needed exhortation, Th1 3:10; therefore he subjoins the conditional clause, "if ye," &c. (Phi 4:1).