71 Admit, therefore, that Christ, as touching His Godhead, cannot be called inferior [to the Father]. Christ speaks to Abraham: “By Myself have I sworn.” Now the Apostle shows that He Who swears by Himself cannot be lower than any. Thus he says, “When God rewarded Abraham with His promise, He swore by Himself, forasmuch as He had none other that was greater, saying, Surely with blessing will I bless you, and with multiplying will I multiply you.” Christ had, therefore, none greater, and for that cause swore He by Himself. Moreover, the Apostle has rightly added, “for men swear by one greater than themselves,” forasmuch as men have one who is greater than themselves, but God has none.
72. Otherwise, if our adversaries will understand this passage as referred to the Father, then the rest of the record does not agree with it. For the Father did not appear to Abraham, nor did Abraham wash the feet of God the Father, but the feet of Him in Whom is the image of the man that shall be. Moreover, the Son of God says, “Abraham saw My day, and rejoiced.” It is He, therefore, Who swore by Himself, [and] Whom Abraham saw.
73. And how, indeed, has He any greater than Himself Who is one with the Father in Godhead? Where there is unity, there is no dissimilarity, whereas between greater and less there is a distinction. The teaching, therefore, of the instance from Scripture before us, with regard to the Father and the Son, is that neither is the Father greater, nor has the Son any that is above Him, inasmuch as in Father and Son there is no difference of Godhead parting them, but one majesty.
Source: On the Christian Faith (De fide) (New Advent)