De Synodis
88 Holy brethren, I understand by ὁμοούσιον God of God, not of an essence that is unlike, not divided but born, and that the Son has a birth which is unique, of the substance of the unborn God, that He is begotten yet co-eternal and wholly like the Father. I believed this before I knew the word ὁμοούσιον but it greatly helped my belief. Why do you condemn my faith when I express it by ὁμοούσιον while you cannot disapprove it when expressed by ὁμοιούσιον? For you condemn my faith, or rather your own, when you condemn its verbal equivalent. Do others misunderstand it? Let us join in condemning the misunderstanding, but not deprive our faith of its security. Do you think we must subscribe to the Samosatene Council to prevent any one from using ὁμοούσιον in the sense of Paul of Samosata? Then let us also subscribe to the Council of Nicæa, so that the Arians may not impugn the word. Have we to fear that ὁμοιούσιον does not imply the same belief as ὁμοούσιον? Let us decree that there is no difference between being of one or of a similar substance. The word ὁμοούσιον can be understood in a wrong sense. Let us prove that it can be understood in a very good sense. We hold one and the same sacred truth. I beseech you that we should agree that this truth, which is one and the same, should be regarded as sacred. Forgive me, brethren, as I have so often asked you to do. You are not Arians: why should you be thought to be Arians by denying the ὁμοούσιον?
89. But you say: 'The ambiguity of the word ὁμοούσιον troubles and offends me.' I pray you hear me again and be not offended. I am troubled by the inadequacy of the word ὁμοιούσιον. Many deceptions come from similarity. I distrust vessels plated with gold, for I may be deceived by the metal underneath: and yet that which is seen resembles gold. I distrust anything that looks like milk, lest that which is offered to me be milk but not sheep's milk: for cow's milk certainly looks like it. Sheep's milk cannot be really like sheep's milk unless drawn from a sheep. True likeness belongs to a true natural connection. But when the true natural connection exists, the ὁμοούσιον is implied. It is a likeness according to essence when one piece of metal is like another and not plated, if milk which is of the same colour as other milk is not different in taste. Nothing can be like gold but gold, or like milk that did not belong to that species. I have often been deceived by the colour of wine: and yet by tasting the liquor have recognized that it was of another kind. I have seen meat look like other meat, but afterwards the flavour has revealed the difference to me. Yes, I fear those resemblances which are not due to a unity of nature.
90. I am afraid, brethren, of the brood of heresies which are successively produced in the East: and I have already read what I tell you I fear. There was nothing whatever suspicious in the document which some of you, with the assent of certain Orientals, took on your embassy to Sirmium to be there subscribed. But some misunderstanding has arisen in reference to certain statements at the beginning which I believe you, my holy brethren, Basil, Eustathius, and Eleusius, omitted to mention lest they should give offense. If it was right to draw them up, it was wrong to bury them in silence. But if they are now unmentioned because they were wrong we must beware lest they should be repeated at some future time. Out of consideration for you I have hitherto said nothing about this: yet you know as well as I do that this creed was not identical with the creed of Ancyra. I am not talking gossip: I possess a copy of the creed, and I did not get it from laymen, it was given me by bishops.
91. I pray you, brethren, remove all suspicion and leave no occasion for it. To approve of ὁμοιούσιον, we need not disapprove of ὁμοούσιον. Let us think of the many holy prelates now at rest: what judgment will the Lord pronounce upon us if we now say anathema to them? What will be our case if we push the matter so far as to deny that they were bishops and so deny that we are ourselves bishops? We were ordained by them and are their successors. Let us renounce our episcopate, if we took its office from men under anathema. Brethren, forgive my anguish: it is an impious act that you are attempting. I cannot endure to hear the man anathematized who says ὁμοούσιον and says it in the right sense. No fault can be found with a word which does no harm to the meaning of religion. I do not know the word ὁμοιούσιον, or understand it, unless it confesses a similarity of essence. I call the God of heaven and earth to witness, that when I had heard neither word, my belief was always such that I should have interpreted ὁμοιούσιον by ὁμοούσιον. That is, I believed that nothing could be similar according to nature unless it was of the same nature. Though long ago regenerate in baptism, and for some time a bishop, I never heard of the Nicene creed until I was going into exile, but the Gospels and Epistles suggested to me the meaning of ὁμοούσιον and ὁμοιούσιον. Our desire is sacred. Let us not condemn the fathers, let us not encourage heretics, lest while we drive one heresy away, we nurture another. After the Council of Nicæa our fathers interpreted the due meaning of ὁμοούσιον with scrupulous care; the books are extant, the facts are fresh in men's minds: if anything has to be added to the interpretation, let us consult together. Between us we can thoroughly establish the faith, so that what has been well settled need not be disturbed, and what has been misunderstood may be removed.
92. Beloved brethren, I have passed beyond the bounds of courtesy, and forgetting my modesty I have been compelled by my affection for you to write thus of many abstruse matters which until this our age were unattempted and left in silence. I have spoken what I myself believed, conscious that I owed it as my soldier's service to the Church to send to you in accordance with the teaching of the Gospel by these letters the voice of the office which I hold in Christ. It is yours to discuss, to provide and to act, that the inviolable fidelity in which you stand you may still keep with conscientious hearts, and that you may continue to hold what you hold now. Remember my exile in your holy prayers. I do not know, now that I have thus expounded the faith, whether it would be as sweet to return unto you again in the Lord Jesus Christ as it would be full of peace to die. That our God and Lord may keep you pure and undefiled unto the day of His appearing is my desire, dearest brethren.
Source: On the Councils, or the Faith of the Easterns (New Advent)