Yet we teach from the Scriptures that God is one in essence, and hence that the essence both of the Son and of the Spirit is unbegotten, but inasmuch as the Father is first in order, and from himself begot his wisdom, as has just been said, he rightly deemed the beginning and fountainhead of the whole of divinity. Thus God without particularization is unbegotten; and the Father also in respect to his person is unbegotten. They also foolishly think they may conclude from our statement that we have set up a quaternity, for they falsely and calumniously ascribe this fiction of their own brain to us, as if we pretended that three persons came forth by derivation from one essence. On the contrary, it is clear from our writings that we do not separate the persons from the essence, but we distinguish among them while they remain within it.
… For although the essence does not enter into the distinction as a part or a member of the Trinity, nevertheless the persons are not without it, or outside it; because the Father, unless he were God, could not have been the Father; and the Son could not have been the Son, unless he were God. Therefore we say that deity in an absolute sense exists of itself; whence likewise we confess that the Son since he is God, exists of himself, but not in respect of his Person; indeed since he is the Son, we say that he exists from the Father. Thus his essence is without beginning; while the beginning of his person is God himself.
~ Institutes I. XIII. 25