14 I have been compelled to dwell briefly on this, lest we should forget our Lord Jesus Christ is being treated of as a Person of two natures, since He, Who was abiding in the form of God, took the form of a servant, in which He was obedient even unto death. The obedience of death has nothing to do with the form of God, just as the form of God is not inherent in the form of a servant. Yet through the Mystery of the Gospel Dispensation the same Person is in the form of a servant and in the form of God, though it is not the same thing to take the form of a servant and to be abiding in the form of God; nor could He Who was abiding in the form of God, take the form of a servant without emptying Himself, since the combination of the two forms would be incongruous.
Yet it was not another and a different Person Who emptied Himself and Who took the form of a servant. To take anything cannot be predicated of some one who is not, for he only can take who exists. The emptying of the form does not then imply the abolition of the nature: He emptied Himself, but did not lose His self: He took a new form, but remained what He was. Again, whether emptying or taking, He was the same Person: there is, therefore, a mystery, in that He emptied Himself, and took the form of a servant, but He does not come to an end, so as to cease to exist in emptying Himself, and to be non-existent when He took.
The emptying availed to bring about the taking of the servant's form, but not to prevent Christ, Who was in the form of God, from continuing to be Christ, for it was in very deed Christ Who took the form of a servant. When He emptied Himself to become Christ the man, while continuing to be Christ the Spirit, the changing of His bodily fashion, and the assumption of another nature in His body, did not put an end to the nature of His eternal divinity, for He was one and the same Christ when He changed His fashion, and when He assumed our nature.
Source: On the Trinity (New Advent)