Book 1 Chapter 13 Sections 25-29
The war continues. Calvin moves from refuting anti-Trinitarian arguments from Scripture to refuting their arguments from extra-biblical sources. As he does so, he provides perhaps the clearest expression of the Trinity I’ve ever read. Isn’t that how it is? The Church grows and is strengthened when it is tested by heresy. This is all throughout history… what a reminder to us in the midst of so much confusion and outright blasphemy!
The essence of all three Persons is deity, not just the Father.
… although the essence does not enter into the distinction as a part of 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.
… those who want to make a Trinity of these three – Essence, Son, and Spirit – are plainly annihilating the essence of the Son and the Spirit; otherwise the parts joined together would fall apart, and this is faulty in any distinction. Finally, if Father and God were synonymous, thus would the Father be the deifier [sic]; nothing would be left in the Son but a shadow; and the Trinity would be nothing else but the conjunction of the one God with two created things.
The essence of deity is shared by all three Persons; all three Persons are properly God. To be the Son of God is to be God, and to be the Spirit of God is to be God. But, as Calvin infers above, we cannot comprehend all of God in any one of the three Persons. God is not totally synonymous with any of the three; He is more fully understood in the three.
Anti-Trinitarians quote Irenaeus and Tertullian to give their viewpoint some theological cred. Calvin soundly defeats both of these appeals to authority. Instead, he shows how both of these church fathers continually argued for the the deity of Christ. Irenaeus said,
…he who in Scripture is called God in an absolute and undifferentiated sense is in truth the only God, and that Christ indeed is called God in an absolute sense.
Similarly, Tertullian, in refuting a heretic, explicitly declares that there are as many names for God as there are persons.
Finally, Calvin appeals to the judgment at Nicaea, where orthodox doctrine concerning the Trinity was established. Arius was soundly defeated, and Augustine was triumphant. The Trinity as a doctrine was embraced by the whole of the church fathers, and thus should be embraced by us today.