You are correct Rivergang, with the Edict of Milan Constantine made it safe to be a Christian, or of any other religion !
Theodosius later made Christianity the Religion of choice of the Empire. He was a Christian, and believed the Trinity to be an accurate Doctrine, so that became part of the orthodoxy he imposed.
He seemed tolerant of other religions existence, but did not encourage them ! For that reason from his time on it was expedient for people to appear to be Christian, hence at the remains of a Roman Villa near where I live we have the wealthy owner's Christian Chapel boldly displayed as part of the Villa, but by a stairway, well hidden no doubt, you could go down to a Temple below the Chapel that was used to worship Mithras in secret. Fascinating !