How Constantine received Instruction, and read the Sacred Scriptures.
Life of Constantine, with Orations · Eusebius of Caesarea
These things were done shortly afterwards. But at the time above specified, being struck with amazement at the extraordinary vision, and resolving to worship no other God save Him who had appeared to him, he sent for those who were acquainted with the mysteries of His doctrines, and enquired who that God was, and what was intended by the sign of the vision he had seen. They affirmed that He was God, the only begotten Son of the one and only God: that the sign which had appeared was the symbol of immortality, and the trophy of that victory over death which He had gained in time past when sojourning on earth. They taught him also the causes of His advent, and explained to him the true account of His incarnation. Thus he was instructed in these matters, and was impressed with wonder at the divine manifestation which had been presented to his sight. Comparing, therefore, the heavenly vision with the interpretation given, he found his judgment confirmed; and, in the persuasion that the knowledge of these things had been imparted to him by Divine teaching, he determined thenceforth to devote himself to the reading of the Inspired writings.
EusVC 3.32.1