From his Letter to Cledonius (taken from Christology of the Later Fathers, pp. 223f):
But if anyone who thinks we have spoken rightly on this subject reproaches us with holding communion with heretics, let him prove that we are open to this charge, and we will either convince him or retire. But it is not safe to make any innovation before judgment is given, especially in a matter of such importance, and connected with so great issues. We have protested and continue to protest this before God and men. And not even now, be well assured, should we have written this, if we had not seen that the Church was being torn asunder and divided, among their other tricks, by their present synagogue of vanity. But if anyone when we say and protest this, either from some advantage they will thus gain, or through fear of men, or monstrous littleness of mind, or through some neglect of pastors and governors, or through love of novelty and proneness to innovations, rejects us as unworthy of credit, and attaches himself to such men, and divides the noble body of the Church, he shall bear his judgment, whoever he may be, and shall give account to God in the day of judgment. But if their long books, and their new Psalters, contrary to that of David, and the grace of their metres, are taken for a third Testament, we too will compose Psalms, and will write much in metre. For we also think we have the spirit of God, if indeed this is a gift of the Spirit, and not a human novelty. This I will that thou declare publicly, that we may not be held responsible, as overlooking such an evil, and as though this wicked doctrine received food and strength from our indifference.