3 For this reason we believe also in His Son, that is to say, God the Father Almighty's, “His Only Son, our Lord.” When you hear of the Only Son of God, acknowledge Him God. For it could not be that God's Only Son should not be God. What He is, the same did He beget, though He is not that Person Whom He begot. If He be truly Son, He is that which the Father is; if He be not that which the Father is, He is not truly Son. Observe mortal and earthly creatures: what each is, that it engenders.
Man besets not an ox, sheep besets not dog, nor dog sheep. Whatever it be that begets, that which it is, it begets. Hold ye therefore boldly, firmly, faithfully, that the Begotten of God the Father is what Himself is, Almighty. These mortal creatures engender by corruption. Does God so beget? He that is begotten mortal generates that which himself is; the Immortal generates what He is: corruptible begets corruptible, Incorruptible begets Incorruptible: the corruptible begets corruptibly, Incorruptible, Incorruptibly: yea, so begets what Itself is, that One begets One, and therefore Only.
You know, that when I pronounced to you the Creed, so I said, and so you are bounden to believe; that we “believe in God the Father Almighty, and in Jesus Christ His Only Son.” Here too, when you believe that He is the Only, believe Him Almighty: for it is not to be thought that God the Father does what He will, and God the Son does not what He will. One Will of Father and Son, because one Nature. For it is impossible for the will of the Son to be any whit parted from the Father's will. God and God; both one God: Almighty and Almighty; both One Almighty.
Source: On the Creed: A Sermon to Catechumens (New Advent)