Athanasius the Great

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(De Dec. Nic. Syn.) The holy Scriptures by the name of Son set forth two meanings; one similar to that spoken of in the Gospel, He gave to them power that they should become the sons of God; another according to which Isaac is the son of Abraham. Christ is not then simply called a Son of God, but the article is prefixed, that we should understand that He alone is really and by nature the Son; and hence He is said to be the Only begotten. For if according to the madness of Arius He is called Son, as they are called who obtain the name through grace, He will seem in no way to differ from us. It remains therefore that in another respect we must confess Christ to be the Son of God, even as Isaac is acknowledged to be the son of Abraham. For that which is naturally begotten of another, and takes not its origin from any thingbesides nature, accounts a son. But it is said, Was then the birth of the Son with suffering as of a man? By no means. God since He cannot be divided is without suffering the Father of the Son. Hence He is called the Word of the Father, because neither is the word of man even produced with suffering, and since God is by nature one, He is the Father of one only Son, and therefore it is added, Beloved. For when a man has only one son, he loves him very much, but if he becomes father of many, his affection is divided by being distributed.