That he wrote to Eusebius respecting Easter, and respecting Copies of the Holy Scriptures
Ever careful for the welfare of the churches of God, the emperor addressed me personally in a letter on the means of providing copies of the inspired oracles, and also on the subject of the most holy feast of Easter. For I had myself dedicated to him an exposition of the mystical import of that feast; and the manner in which he honored me with a reply may be understood by any one who reads the following letter.
Source: Life of Constantine (New Advent)