Should you, however, ask in what way Deity is mingled with humanity, you will have occasion for a preliminary inquiry as to what the coalescence is of soul with flesh. But supposing you are ignorant of the way in which the soul is in union with the body, do not suppose that that other question is bound to come within your comprehension; rather, as in this case of the union of soul and body, while we have reason to believe that the soul is something other than the body, because the flesh when isolated from the soul becomes dead and inactive, we have yet no exact knowledge of the method of the union, so in that other inquiry of the union of Deity with manhood, while we are quite aware that there is a distinction as regards degree of majesty between the Divine and the mortal perishable nature, we are not capable of detecting how the Divine and the human elements are mixed up together.
The miracles recorded permit us not to entertain a doubt that God was born in the nature of man. But how— this, as being a subject unapproachable by the processes of reasoning, we decline to investigate. For though we believe, as we do, that all the corporeal and intellectual creation derives its subsistence from the incorporeal and uncreated Being, yet the whence or the how, these we do not make a matter for examination along with our faith in the thing itself. While we accept the fact, we pass by the manner of the putting together of the Universe, as a subject which must not be curiously handled, but one altogether ineffable and inexplicable.
Source: The Great Catechism (New Advent)