Spirit of God, Spirit of Christ

The New Testament will not hold still on this. Spirit of God, Spirit of Christ, Spirit of Jesus - the terms are interchanged in several verses, sometimes within a single sentence.
Paul writes that you are in the Spirit if the Spirit of God dwells in you, and then says anyone who does not have the Spirit of Christ does not belong to him. Same Spirit, two names, no explanation offered.
That is not sloppiness. It is the same pattern the Old Testament already had with the two Yahweh figures - distinct, and yet not separable. The Spirit is identified with God without being collapsed into the Father or the Son.
Scripture links:
- Romans 8:9-10 - the Spirit of God, and the Spirit of Christ.
- Galatians 4:4-6 - God sent the Spirit of his Son into our hearts, crying Abba, Father.
The vocabulary was settled long before the doctrine had a name.