Jimb quoted John 15.26:
“When the Helper comes, whom I will send to you from the Father, namely, the Spirit of truth who comes from the Father, He will testify about Me (NASB).
I agree with
@jimb that this verse is evidence for the Trinity, though I would say it is just
some of the evidence for the Trinity. In the following, I will explain how John 15.26 is some evidence for the Trinity.
First, Jesus mentions the Helper/Advocate and that this Helper would testify of Jesus. The title (Helper) and the action (to testify) together imply that Jesus is speaking of a person. Now, anybody can call himself an advocate and pretend to testify about something, so Jesus tells them the origin of the Helper, where the Helper comes from.
Second, he tells them the origin of the Helper by stating he is the ‘Spirit of truth who comes from the Father.’ The Greek term translated as ‘comes from’ is ἐκπορεύεται or
ekporeúetai, which is traditionally translated into English as ‘proceeds from.’ If I state that I proceeded from a room, you know that I as a creature had to enter that room from elsewhere and then leave it. On the other hand, nowhere in the Bible does it state that the Spirit came from elsewhere, entered the Father, and then proceeded from the Father. The implication is that the Spirit originated from the Father by procession. It is like saying the Spirit was ‘spirated’ or breathed out by God. If God the Father is not a composite being, in other words, not a creature, then his Spirit, his own Breath as it were, must be exactly
what he is—God, the divine. This is the implication. Altogether, the obvious personhood of the Spirit and the implication of his divine essence is evidence that supports the Trinity.