25. The essential likeness conformed to the Father’s essence in kind is also taught to be identical in time: lest He who is the image of God, who is the Word, who is God with God in the beginning, who is like the Father, by the insertion of times between Himself and the Father should not have in Himself in perfection that which is both image, and Word, and God. For if He be proclaimed to be younger in time, He has lost the truth of the image and likeness: for that is no longer likeness which is found to be dissimilar in time. For that very fact that God is Father prevents there being any time in which He was not Father: consequently there can be no time in the Son’s existence in which He was not Son. Wherefore we must neither call the Father older than the Son nor the Son younger than the Father: for the true meaning of neither name can exist without the other.

XII. “And if any one attributes the timeless substance (i.e. Person) of the Only-begotten Son derived from the Father to the unborn essence of God, as though calling the Father Son: let him be anathema17    Substantia is in this passage used as the equivalent of Person. The word was used by Tertullian in the sense of οὐσία, and this early Latin use of the word is the use which eventually prevailed. The meaning of the word in Hilary is influenced by its philological equivalent in Greek. At the beginning of the fourth century ὑπόστασις was used in the same sense as οὐσία. The latter word meant ‘reality,’ the former word ‘the basis of existence.’ Athanasius, however, began the practice of restricting ὑπόστασις to the divine Persons. Hilary consequently here uses substantia in this new sense of the word ὑπόστασις. The Alexandrine Council of 362 sanctioned as allowable the use of ὑπόστασις in the sense of Person, and by the end of the century the old usage practically disappeared..”

25. Filius ut Patris imago, non est eo junior. Ex eo quod Deus semel pater, semper fuit pater, semper et Filius.---Similitudo essentiae configurata in genere, indifferens quoque docetur in tempore: ne qui imago est Dei, qui Verbum est, qui Deus est apud Deum in principio, qui similis est Patri, interjecto inter se ac Patrem tempore, non perfectum in se habeat quod et imago est, et Verbum est, et Deus est. Si enim tempore junior praedicatur, amisit et imaginis et similitudinis veritatem: quia similitudo jam non sit, quae dissimilis reperiatur in tempore. Quia ipsum illud, quod Deus pater est, tempus aliquod non potest habere ne Pater sit: ex quo nec in 0499C Filio tempus potest esse ne filius sit. Atque ob id, nec Pater Filio senior, nec Filius Patre junior esse dicendus est: quia utriusque nominis veritas non potest esse sine altero.

476 XII.«Et si quis intemporalem unigeniti filii de Patre substantiam ad innascibilem Dei essentiam referat, quasi Filium Patrem dicens: anathema sit.»