Just what the Council intended this expression to mean is set forth by St. Athanasius as follows: “That the Son is not only like to the Father, but that, as his image, he is the same as the Father; that he is of the Father; and that the resemblance of the Son to the Father, and his immutability, are different from ours: for in us they are something acquired, and arise from our fulfilling the divine commands. Moreover, they wished to indicate by this that his generation is different from that of human nature; that the Son is not only like to the Father, but inseparable from the substance of the Father, that he and the Father are one and the same, as the Son himself said: ‘The Logos is always in the Father, and, the Father always in the Logos,’ as the sun and its splendour are inseparable.”
Henry R. Percival, “Excursus on the Word Homousios,” in A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, Second Series: The Seven Ecumenical Councils, ed. Philip Schaff and Henry Wace, vol. 14 (New York: Charles Scribner’s Sons, 1900), 3–4.