The same logic that we have seen with the Father and the Son applies also to the Holy Spirit: he is who he is for Trinitarian identity in which he exists together with the Father and the Son, he freely steps into the history of salvation and does what he does. The work of the Spirit is closely linked to that of the Son at every point. It is the Spirit who brings about the Son’s incarnation by causing his conception in the womb of the virgin. It is the Spirit who anoints and empowers the Son in his messianic mission. And the Spirit is finally, at Pentecost, poured out on all flesh only when the Son’s work is completed. The Spirit’s work is to indwell believers, applying the work of Christ directly and personally to them. He is who he is as the eternal Spirit, and he does what he does in salvation history as the Spirit of Pentecost.

   Because the eternal Son became the incarnate Son, we had much to say about his sonship. Tracing the line back from his appearance in Bethlehem is how we learned anything about the Trinity at all, for this is the central event in which God revealed that he had a Son. We had relatively less to say about the Father, and most of it was directly connected with the Son: the Father is the person of the Trinity who is obviously at the other end of the relationship that makes the Son the Son. But we have least of all to say about the eternal divine person who is the Holy Spirit, not because he is any less God, or any less a person, or any less related to the other persons of the Trinity. He is all those things, just as fully as the Father and the Son are. But his self-revelation is less direct than the Son’s, and his relationship to the other persons is not as immediately evident as the Son’s and Father’s, whose mutual relationship is built into their very names. We should avoid the urge to fabricate more concrete things than have actually been revealed about the Spirit or to pretend that our knowledge of the Spirit’s corner of the Trinitarian triangle is as intricately detailed and elaborated as the Son’s. 

The Deep Things of God - Fred Sanders