On the Opinion of Dionysius is a letter to a person engaged in discussion with Arians, who were openly finding fault with the Definition of Nicaea, and especially with the word 'Co-essential'. The Letter concerns Dionysius, then Bishop of Alexandria, showing that he too was against the Arian heresy, like the Synod of Nicaea, and that the Arians libel him in claiming him as on their side.