The trinity and incarnation

Not sure how to ask this question correctly but, here goes:

In catholic theology of the trinity: When speaking of the father, son, and holy spirit, all refer to the same ‘substance’ of God (they all share the same substance). correct?

When ‘God the Son’ became incarnate, he assumed a human nature (became Jesus of Nazareth) …
but when God assumes a human nature, how is it that only that one specific person in the trinity can assume the human nature without the other persons also sharing that same human nature… ? The divine person cannot be separated from the substance of God (he is what he is because of that one substance)?

Note: I’m not trying to tend towards a modalist understanding of the trinity… I’m just not clear as to what aspect of the divine person can assume the human nature if that persons very nature is from that one substance…


Here is St Thomas Aquinas answer to that:
" As was said above (Article 1), assumption implies two things, viz. the act of assuming and the term of assumption. Now the act of assumption proceeds from the Divine power, which is common to the three Persons, but the term of the assumption is a Person, as stated above (Article 2). Hence what has to do with action in the assumption is common to the three Persons; but what pertains to the nature of term belongs to one Person in such a manner as not to belong to another; for the three Persons caused the human nature to be united to the one Person of the Son."
A more precise and correct rendering of God assuming human nature is: The Second Person of the Trinity assumed human nature. It was a Person that assumed the nature of man not the Being.


So when we say that the persons of the Trinity are all of the same substance, there is sort of some background that needs to be understood by what that means. Tertullian was addressing the heresy of subordinationism to explain how the Son is not a different or lesser god than the Father. What he was saying is that the Bible clearly states two things. First, there is only one God. But it also shows that the Father, Son, and Holy Spirit are God, and that these three are distinct from one another. To explain this concept he used the legal terminology of the day to provide an analogy that could be readily grasped. In law, there are only two things, there are persons and property. He described divinity itself as a property (substance) that all three persons of the Trinity share. In other words, the Father possesses divinity itself, as does the Son, and the Holy Spirit. Whatever it is to be divine, all three possess it. Since there is only one God, there cannot be more than one property of divinity. In other words, whatever divinity is, the Father fully possesses that characteristic. The Son fully possesses it, as does the Holy Spirit. The Son did not have a lesser divinity, nor did he give up his diving nature, but during the incarnation, he took upon himself the full nature of humanity as well.

I think the key truth that explains the OP’s question is there - in the truth of the distinction of Persons. The Father is fully God, but He is not and cannot be the Son. The Son is fully God, but cannot be the Father nor the Spirit. The Spirit is fully God, but is not the Son nor can He be the Son. In the distinction of the three Persons lies the property that enables the Son to uniquely assume a human nature to Himself and to Himself only, while the Trinity remains fully God in three (distinct) Persons.

To speculate a bit: The Father could assume another nature into Himself, I suppose, if it were good and true to do so. The Father possibly could also assume a human nature if He determined it good and needful to do so. But He could and did allow the Son and the Son alone to do that. If you think a bit about it, it is most fitting for the Son, as Son, to be the One to do the Son’s mission from the Father - as it was most fitting for the Father to give this mission to the Son and to the Son alone - precisely because the Father IS Father, and the Son IS Son, and the Spirit, meanwhile, waited for His unique mission in salvation history to come, after the Son with His glorified human nature ascended to His place of glory with the Father and the Spirit.

The Spirit - as Spirit - is most fitting for His mission (as divine Spirit) to become Spirit in, and with, the Church into eternity, sent by the Father and the Son, after they had worked their divine Being and Presence on behalf of created humanity.

Only God can claim to have always been. The three Divine Persons can claim this state f being. I think on our side of existence to be able to contemplate being without a beginning is perceived as incomprehensible unity. Thus one substance.

what if there were four persons that could claim to have always been? Wouldn’t we think they were all of one substance? All of them God? :thinking:

When the Son/Word of God is sent by God the Father…

254 The divine persons are really distinct from one another . “God is one but not solitary.”

86 “Father”, “Son”, “Holy Spirit” are not simply names designating modalities of the divine being, for they are really distinct from one another: "He is not the Father who is the Son, nor is the Son he who is the Father, nor is the Holy Spirit he who is the Father or the Son."87 They are distinct from one another in their relations of origin: "It is the Father who generates, the Son who is begotten, and the Holy Spirit who proceeds."88 The divine Unity is Triune.

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