45 But in us the preacher of the Gospel by the Spirit of Christ Who spoke through him, instils the knowledge of this His proper nature when he says, Who, being in the form of God, thought it not a thing to grasp at that He was equal with God, but emptied Himself, taking the form of a servant. For He, Whom God had sealed, could be naught else than the form of God, and that which has been sealed in the form of God must needs present at the same time imaged forth within itself all that God possesses.
And for this cause the Apostle taught that He Whom God sealed is God abiding in the form of God. For when about to speak of the Mystery of the body assumed and born in Him, he says, He thought it not a thing to grasp at that He was equal with God, but emptied Himself, taking the form of a servant. As regards His being in the form of God, by virtue of God's seal upon Him, he still remained God. But inasmuch as He was to take the form of a servant and become obedient unto death, not grasping at His equality with God, He emptied Himself through obedience to take the form of a slave.
And He emptied Himself of the form of God, that is, of that wherein He was equal with God— not that He regarded His equality with God as any encroachment,— although He was in the form of God and equal with God and sealed by God as God.
Source: On the Trinity (New Advent)