"Strong (pl.) in [the] faith".
It comes from I Peter V, 8-9: "(...) adversarius vester diabolus tamquam Leo rugiens circuit, quærens quem devoret: cui resistite fortes in fide" (Vulgate), "(...) Your adversary the devil, as a roaring lion, walketh about, seeking whom he may devour: whom resist stedfast in the faith" (KJV).