The objection
If the Father is God and the Son is God, the Father would have to be identical with the Son. If both are identical with the same God, they cannot be two different persons. This seems to follow necessarily from identity's transitivity.
1. The logical rule is correct
If A is numerically identical with B in every respect and C likewise with B, then A and C are identical. Trinitarian doctrine needs no exception to this rule. An answer simply claiming that logic does not apply to God would destroy every definite statement about God: “God is faithful” could then also be compatible with “God is not faithful.”
The point to be examined is the translation of theological sentences into equality signs. Not every German “is” designates numerical identity. “Paul is a human” says something about his nature; “Paul is the author of this letter,” by contrast, identifies him under a particular description.[1]
2. John 1,1 requires a distinction of meaning
The Word was with God, and the Word was God. The first clause distinguishes the Word from the one with whom it is. The second does not withdraw this distinction but says what the Word is in its being. Automatically translating both occurrences as the same personal name would tear apart the sentence's context.
“The Son is God” therefore does not mean: The Son is the same person as the Father. Nor does it mean merely: He is somehow godlike. It designates his full deity with real personal distinction.
3. Predication does not yet explain everything
The sentence “Father and Son possess divine nature” alone would also be compatible with two gods if this nature occurred in several separate instances as humanity does. A further statement therefore belongs to Trinitarian doctrine: Divine being is numerically one and undivided. Father and Son are not two instances of the kind “God.”[2]
This answers two different questions: Who is distinguished from the others? Father, Son and Spirit. What is their divine being? Not divinity three times over, but the one uncreated being. This distinction is no complete mechanical model of God; but it determines which statement must actually be examined for contradiction.
4. The objection must not refute a different doctrine
A contradiction would be that God is one person and three persons in the same meaning, or one being and three beings in the same sense. We do not assert this. The formula instead holds different scriptural statements together: one undivided deity, three real personal relationships.
Anyone seeking to derive a formal contradiction must additionally justify that every one divine being can necessarily be exactly one person. This premise does not already come from the law of identity. It would itself be a metaphysical thesis about God—and would have to be justified against his revelation.
5. What Leibniz and substitution of names actually yield
Leibniz's identity principle remains valid: What is strictly the same person cannot simultaneously be father and nonfather of the same son. The differing fatherhood therefore proves that Father and Son are not the same person. Precisely this is confessed by Trinitarian doctrine. The further inference to two divine beings requires an additional thesis: Every personal difference must be a separate instance of divine nature. This thesis does not follow from the identity principle. Conversely, it would be contradictory first to strictly equate Father and Son with a single individual and then assert their difference. A viable explanation must therefore define its statements of being more precisely; the word “relationship” does not replace this work.
“God became human,” too, requires a precise referent: The one who became human is true God, namely the Son. The sentence asserts neither “every divine person became human” nor “divine nature became human nature.” Thomas already distinguishes this concrete use of “God” from the abstract designation of being.[3] “God” therefore cannot be replaced by “Father” here. Satisfying the same predicate “is God” does not turn the persons into interchangeable proper names.
Biblical texts in context
Sources and further study
- Norman L. Geisler, Baker Encyclopedia of Christian ApologeticsArticle “Trinity”: God's unity, distinction of persons and consistency. The distinction between identity and predication of being is expressly developed here.
- Thomas Aquinas, Summa theologiaeI, q. 39, a. 1–3: unity of being and distinction of persons.
- Thomas Aquinas, Summa theologiaeI, q. 39, a. 4: The concrete divine name can stand for a person; being and person must not be linguistically interchanged.