That he wrote a Letter in Condemnation of Idolatry
Moreover, the emperor, who continually made progress in piety towards God, dispatched an admonitory letter to the inhabitants of every province, respecting the error of idolatry into which his predecessors in power had fallen, in which he eloquently exhorts his subjects to acknowledge the Supreme God, and openly to profess their allegiance to his Christ as their Saviour. This letter also, which is in his own handwriting, I have judged it necessary to translate from the Latin for the present work, in order that we may hear, as it were, the voice the emperor himself uttering these sentiments in the audience of all mankind.
Source: Life of Constantine (New Advent)