Excerpt from the book Meditations on Christian Dogma, Volume I - One of the great mysteries of our faith is the hypostatic union — Jesus being both God and Man. But it was necessary for our salvation that God should take on the form of man while not giving up any of His divinity. Because man sinned, so man must atone for the sin. But because sin could only be atoned for by a perfect divinity, God needed to take on the form of man to atone for our sins.