Three Distinct - Not Two Equal
You, however, are not in the flesh but in the Spirit, if in fact the Spirit of God dwells in you. Anyone who does not have the Spirit of Christ does not belong to him. (ESV)
ὑμεῖς δὲ οὐκ ἐστὲ ἐν σαρκὶ ἀλλὰ ἐν πνεύματι εἴπερ πνεῦμα θεοῦ οἰκεῖ ἐν ὑμῖν εἰ δέ τις πνεῦμα Χριστοῦ οὐκ ἔχει οὗτος οὐκ ἔστιν αὐτοῦ
God and Christ are both genitive with the head noun Spirit πνεῦμα θεοῦ...πνεῦμα Χριστοῦ. Spirit is also used without a genitive, πνεύματι. All are anarthrous; "the" Spirit of God and "the" Spirit of Christ add something not present in the text. Understanding Paul intends to describe each as distinct is also seen in how each is used to express a different relationship to the believer.
- Spirit -
δὲ οὐκ ἐστὲ ἐν σαρκὶ ἀλλὰ ἐν πνεύματι You now are not in flesh but in Spirit.
- Spirit of God -
πνεῦμα θεοῦ οἰκεῖ ἐν ὑμῖν Spirit of God dwells in you.
- Spirit of Christ -
πνεῦμα Χριστοῦ οὐκ ἔχει Spirit of Christ not have
Finally, since the most common use of the article is anaphoric1by omission the composition prevents understanding Paul intends to equate either Spirit of God or Spirit of Christ with Spirit. On this basis alone it would be wrong to equate God and Christ. The text has been composed such that both Spirit of God and Spirit of Christ are described as separate and distinct from Spirit.
Spirit πνεύματι - Spirit of God πνεῦμα θεοῦ - Spirit of Christ πνεῦμα Χριστοῦ
The three uses of Spirit have been arranged chiastically centered on Spirit of God dwells in you which is introduced with the conditional εἴπερ. This word is only used by Paul (cf. Romans 3:30, 8:9, 8:17, 1 Corinthians 8:5, 15:15, 2 Thessalonians 1:6) and is absent from the LXX.
In discussing conditional sentences, Daniel B. Wallace places Romans 8:9 in the the category of "Assumed True for the Sake of Argument" and gives this explanation:
Here the conditional particle is a spin-off of εἰ, strengthening the ascensive force. This looks very much like 1 Thessalonians 4:14-i.e., it too seems to be a "responsive" condition. The audience would most likely respond along these lines: "If the Spirit of God dwells in us? Of course he does! And this means that we are not in the flesh but in the Spirit? Remarkable!"2
The "assumed true" aspect of εἴπερ is better expressed as since God's Spirit dwells in you, and the three distinct uses of Spirit describe three distinct conditions:
A You now are not in flesh but in Spirit
ὑμεῖς δὲ οὐκ ἐστὲ ἐν σαρκὶ ἀλλὰ ἐν πνεύματι
B Since Spirit of God dwells in you
εἴπερ πνεῦμα θεοῦ οἰκεῖ ἐν ὑμῖν
A' If now anyone Spirit of Christ does not have he does not belong to him
εἰ δέ τις πνεῦμα Χριστοῦ οὐκ ἔχει οὗτος οὐκ ἔστιν αὐτοῦ
The opening begins with a negative statement, you are not in flesh.... The negative in this case is a positive condition ...in Spirit. The verse begins by stating both sides of being in Spirit. The condition of being in Spirit leads to the second: Spirit of God dwells in you. Together these indicate a relationship between Spirit and God's Spirit. The verse ends by stating a negative condition Spirit of Christ not have and the negative results not belong to him.
Since Spirit of Christ is placed in a negative condition, does not have, the opposite does have is is not found in this verse.3One may assume does have also means does belong, but clearly Paul chose not to say anyone who has Spirit of Christ does belong to him.
The significance of the negative condition is not in either have ἔχει or not have, οὐκ ἔχει. With respect to Christ, the positive condition is to be "in" ἐν. For example, compare 8:9 with 8:1:
(8:1) There is therefore now no condemnation for those who are in Christ Jesus
(8:9a) You now are not in flesh but in Spirit
(8:9b) since Spirit of God dwells in you
(8:9c) If now anyone does not have Spirit of Christ he does not belong to him
The typical Pauline term for those who have come to faith in the Gospel is to be in Christ, ἐν Χριστῷ or in Christ Jesus, ἐν Χριστῷ Ἰησοῦ, not in Spirit, ἐν πνεύματι. Therefore what Paul says in 8:9 looks back to 8:1 which demands understanding in Spirit means the same as in Christ Jesus or in Christ and so, Spirit of God dwells in you. Again, each of the three must be seen as distinct from the other. Despite being distinct, the three describe one condition.
Like the answer to the question who raised Jesus from the dead, the answer to how does one who believes the Gospel experience new life while still in the body, is three-fold. It is not only Spirit. It is not only Spirit of God. It is not only Spirit of Christ. It is three-as-one.
Conclusion
One way of understanding the Trinity is by the deity of Jesus. Yet, we must remember the correct Trinitarian definition of God includes Three, Jesus and Father and Spirit, not one. This means "God" should not be used of just one of the Three, unless it is qualified to distinctly apply to just one of the Three (eg. God and Father of our Lord Jesus Christ or our God and Savior Jesus Christ).
The essence of the Trinity is found in passages such as Romans 8 which describe three distinct in a way which results in one identical condition. Just as the resurrection of Jesus is described in terms of three distinct acting in a single work, so too those who believe the Gospel are described in terms of three distinct: in Christ Jesus and in Spirit and God's Spirit dwells in them.
Significantly, Paul makes no mention of Holy Spirit and the one reference to Father is familial:
14 For all who are led by the Spirit of God are sons of God. 15 For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, “Abba! Father!” 16 The Spirit himself bears witness with our spirit that we are children of God, 18 and if children, then heirs—heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him. (Romans 8)
Paul leaves no doubt of the nature of Spirit, Spirt of God, and Spirit of Christ Jesus in this passage.
1. Daniel B. Wallace, Greek Grammar: Beyond the Basics An Exegetical Syntax of the New Testament, Zondervan, 1996, p. 218
2. Ibid., p. 694
3. When a negative statement is false, it does not logically say anything about the true condition. This is especially where the writer purposely chooses to avoid making explicit the positive condition. After laying out the positive in Spirit and God's Spirit dwells in you, one might expect you have Spirit of Christ and belong to him.