Ambrose, Bishop, to the most blessed Prince and most Christian Emperor Valentinian.
1. As all men who live under the Roman sway engage in military service under you, the Emperors and Princes of the world, so too do you yourselves owe service to Almighty God and our holy faith. For salvation is not sure unless everyone worship in truth the true God, that is the God of the Christians, under Whose sway are all things; for He alone is the true God, Who is to be worshipped from the bottom of the heart; for “the gods of the heathen,” as Scripture says, “are devils.”
Source: Letters (New Advent)