Ambrose of Milan

This is our Faith. Thus did God will that He should be known by all, thus believed the three children, and felt not the fire into the midst whereof they were cast, which destroyed and burnt up unbelievers, whilst it fell harmless as dew upon the faithful, for whom the flames kindled by others became cold, seeing that the torment had justly lost its power in conflict with faith. For with them there was One in the form of an angel, comforting them, to the end that in the number of the Trinity one Supreme Power might be praised. …

Exposition of the Christian Faith 1:4.33 ↗