Library Eusebius of Caesarea Life of Constantine
Book 4
Chapter 24: That he described himself to be a Bishop, in Charge of Affairs External to the Church
Hence it was not without reason that once, on the occasion of his entertaining a company of bishops, he let fall the expression, that he himself too was a bishop,
addressing them in my hearing in the following words: You are bishops whose jurisdiction is within the Church: I also am a bishop, ordained by God to overlook whatever is external to the Church.
And truly his measures corresponded with his words: for he watched over his subjects with an episcopal care, and exhorted them as far as in him lay to follow a godly life.
English translation by Ernest Cushing Richardson, in Nicene and Post-Nicene Fathers, Series II, Vol. I (eds. Philip Schaff and Henry Wace; Buffalo: Christian Literature Publishing Co., 1890). Public domain. Digital text via New Advent Church Fathers.