There is a frequent assumption that unbelievers have about the Trinity. But if you are aware of this deep assumption by those who deny the deity of Christ, you can disarm them. James White has said:
Difference in function does not indicate inferiority of nature.
There is a built-in assumption for many that if Jesus possesses a lesser role than the Father, he must therefore posses a lesser nature. This is not a valid inference. Those who oppose the deity of Christ point to Jesus’ submissive remarks about doing the will of his Father.
For example, Jesus says, “the Father is greater than I am.” They infer from this that Jesus does not share the same nature with the Father. This ignores that the context is talking about their relational roles, not their nature, John 14. Jesus also calls the Father, “My God.” Yet those who oppose the deity of Christ ignore that this is a humble acknowledgment of the incarnate Jesus, modeling for us humility and submissiveness (John 20:17). This exalting affirmation is what we would expect from the Son of God.
Similarly, it is argued, since Jesus is the agent of the Father in many respects such as creation, Jesus cannot be fully God. Regarding the Spirit, they will make the similar false assumption: Since the Spirit is sent by the Father, the Spirit cannot have the same divine nature as the Father. They will look at these statements and make the fallacious leap that difference in function indicates inferiority of nature.
By doing committing this fallacy, they also deny the freedom of the Divine persons to choose their roles. Or to put it another way: they assume that to be truly God, the Son and the Spirit must have the exact same roles as the Father. Do not allow them to accept this assumption.
A simple, but effective, illustration will show that difference in function does not indicate inferiority of nature: A husband and wife will have different roles in a marriage. Wives are to take on the submissive role, but this does not indicate that difference in function requires inferiority of nature. Does the wife have a lesser nature than that of the husband? Not according to Christian anthropology. They both are fully human.
Let’s praise God for the incarnation, which itself presupposes a submissive role that brought about our salvation. We do not worship a unipersonal-unitarian God, but instead a complementary-trinitarian God.