The answer, as we’ve seen in previous chapters, is that Jesus is the second Yahweh, the embodied Yahweh of the Old Testament. But Jesus is not the “Father” Yahweh. He therefore is but isn’t Yahweh. It’s the same with the Spirit. The Spirit is Yahweh, and so he is Jesus as well, but not incarnate or embodied. The Spirit is but isn’t Jesus, just as Jesus is but isn’t Yahweh the Father. The same sort of “two Yahwehs” idea from the Old Testament is found in the New Testament with respect to Jesus and the Spirit. That is the source of Trinitarian theology.
