I. The two-fold nature of Christ set forth
Among all the works of God's mercy, dearly-beloved, which from the beginning have been bestowed upon men's salvation, none is more wondrous, and none more sublime, than that Christ was crucified for the world. For to this mystery all the mysteries of the ages preceding led up, and every variation which the will of God ordained in sacrifices, in prophetic signs, and in the observances of the Law, foretold that this was fixed, and promised its fulfilment: so that now types and figures are at an end, and we find our profit in believing that accomplished which before we found our profit in looking forward to.
In all things, therefore, dearly-beloved, which pertain to the Passion of our Lord Jesus Christ, the Catholic Faith maintains and demands that we acknowledge the two Natures to have met in our Redeemer, and while their properties remained, such a union of both Natures to have been effected that, from the time when, as the cause of mankind required, in the blessed Virgin's womb, “the Word became flesh,” we may not think of Him as God without that which is man, nor as man without that which is God.
Each Nature does indeed express its real existence by actions that distinguish it, but neither separates itself from connection with the other. Nothing is wanting there on either side; in the majesty the humility is complete, in the humility the majesty is complete: and the unity does not introduce confusion, nor does the distinctiveness destroy the unity. The one is passible, the other inviolable; and yet the degradation belongs to the same Person, as does the glory. He is present at once in weakness and in power; at once capable of death and the vanquisher of it. Therefore, God took on Him whole Manhood, and so blended the two Natures together by means of His mercy and power, that each Nature was present in the other, and neither passed out of its own properties into the other.
Source: Sermons (New Advent)