Nicene and Post-Nicene Fathers, Ser. II, Vol I:
The Life of Constantine with Orations of Constantine and Eusebius.: Chapter II

Chapter II.—Farther Remarks on Constantine’s Piety, and his Open Testimony to the Sign of the Cross.

And when we consider that their iniquities were without example, and the atrocities which they dared to perpetrate against the Church such as had never been heard of in any age of the world, well might God himself bring before us something entirely new, and work thereby effects such as had hitherto been never either recorded or observed. And what miracle was ever more marvelous than the virtues of this our emperor, whom the wisdom of God has vouchsafed as a gift to the human race? For truly he maintained a continual testimony to the Christ of God with all boldness, and before all men; and so far was he from shrinking from an open profession of the Christian name, that he rather desired to make it manifest to all that he regarded this as his highest honor, now impressing on his face the salutary sign, and now glorying in it as the trophy which led him on to victory. 3229

Footnotes

Note the explicit testimony of Eusebius here, and compare Prolegomena, under Religious Characteristics.