Emperors Constantine (c. 274–337) and Licinius issued the Edict of Milan in 313, giving full legal recognition to Christianity for the first time. Constantine convened the Council of Nicaea in 325 and began the process that eventually led to the full union of church and state. He was baptized on his deathbed by an Arian, Eusebius of Nicomedia. The following quotations are all from Lactantius, whom Constantine summoned to tutor his son, Crispus.
We now commence this work under the auspices of your name, O mighty Emperor Constantine, who was the first of the Roman rulers to repudiate errors and to acknowledge and honor the majesty of the one and only true God. For when that most happy day had shone upon the world, in which the Most High God raised you to the prosperous height of power, you entered upon a dominion that was both salutary and desirable for all. Lactantius (c. 304–313, W), 7.10.
All fictions have now been hushed, most holy Emperor—ever since the time when the great God raised you up for the restoration of the house of justice and for the protection of the human race. For while you rule the Roman state, we worshippers of God are no longer regarded as accursed and impious. . . . The providence of the supreme Deity has raised you to the imperial dignity so that you might, with true piety, rescind the injurious decrees of others. . . . The powerful right hand of God protects you from all dangers. He bestows on you a quiet and tranquil reign, with the highest congratulations of all men. And not undeservedly has the Lord and Ruler of the world chosen you in preference to all others, by whom He might renew His holy religion. . . . We supplicate Him with daily prayers that He may especially guard you, whom He has wished to be the guardian of the world. Lactantius (c. 304–313, W), 7.221, 222
Constantius also had a son, Constantine. He was a young man of very great worth and well deserving the high station of Caesar. The distinguished comeliness of his figure, his strict attention to all military duties, his virtuous demeanor and singular friendliness—all these had endeared him to the troops and made him the choice of every individual. Lactantius (c. 320, W), 7.308
Constantine Augustus, having assumed the government, made it his first task to restore Christians to the exercise of their worship and to their God. Lactantius (c. 320, W), 7.311.
Constantine was directed in a dream to cause the heavenly sign to be outlined on the shields of his soldiers and so to proceed to battle. He did as he had been commanded. And he marked on their shields the letter X, with a perpendicular line drawn through it and turned round at the top—being the monogram of Christ. Lactantius (c. 320, W), 7.318