Vous avez cherché: semper firmus et fidei (Latin - Anglais)

semper firmus
always strong and strong

firmus et fidelis
firm and loyal

si enim qui per angelos dictus est sermo factus est firmus et omnis praevaricatio et inoboedientia accepit iustam mercedis retributione
for if the word spoken by angels was stedfast, and every transgression and disobedience received a just recompence of reward;

et illi dixerunt ad hieremiam sit dominus inter nos testis veritatis et fidei si non iuxta omne verbum in quo miserit te dominus deus tuus ad nos sic faciemu
then they said to jeremiah, the lord be a true and faithful witness between us, if we do not even according to all things for the which the lord thy god shall send thee to us.