Paul says that we are united to Christ in the likeness of his death through baptism (Rom 6.5). The phrase he uses to describe this union is σύμφυτοι γεγόναμεν, the critical adjective here being the nominative masculine plural of σύμφυτος, symphytos. What does this word mean, and what can it tell us about the nature of this union?
The word σύμφυτος is ordinarily used to describe things which grow together or have been planted together; it is an agricultural word in this sense. Thus, for example in the parable of the sower, some of the seed falls among the thorns and the thorns grow together with it. The verb for "growing together" here is συμφύω, where we get the adjective σύμφυτος from. This definition of the word is not immediately helpful for us to understand Paul's usage, however.
A closely related definition found in the classical Greek literature is "innate, congenital." We seem to be approaching Paul's usage to some extent, but we are not quite there yet.
There is an interesting usage of the adjective in Plato. In a passage from the Phaedrus 246a, Plato speaks of the human soul as analogous to the composite of a pair of winged horses and the charioteer (ἐοικέτω δὴ συμφύτῳ δυνάμει ὑποπτέρου ζεύγους τε καὶ ἡνιόχου). Here we get an understanding of σύμφυτος as a composite, a unity of multiple parts.
This is obviously closer to Paul's understanding of union with Christ in baptism. Effectively what Paul is saying is that we have become one thing with Christ through baptism; now, together with Christ, we form a single thing. In other places in the Pauline corpus, he refers to this one thing as the body of Christ or the church, of whom Christ is the head. But what exactly is the nature of this union?
Paul makes it clear in his discussion of baptism in Rom 6 that the union is a bodily one. It is hard to understand for us, perhaps, but this is the language he uses! He talks of us dying together with Christ, of being buried with him, and of rising from the dead too. He talks about us being in Christ, who has died to sin, and this is supposed to neutralize our sinful bodies (Rom 6.6). To my mind, what Paul is saying is this: our bodies -- which, in light of his Jewish materialist anthropology, means our very selves -- have been united to Christ's body in a mystical way through baptism, so that his history becomes our history, and the sinfulness of our body is neutralized through its union to his body.
In a way, we are returning to the more organic and agricultural definitions of σύμφυτος with this interpretation. We become part of Christ through baptism; we grow with him as a single body; we become innate to him as members of his body.
Some persons would rather see only forensic and juridical categories here and leave it at that. They may want to speak of a "mystical" or "spiritual" union with Christ without positing a bodily union. That all goes against Paul's language, however, which emphasizes strongly the bodily nature of our union with Christ. We are united to Christ in a bodily way because we take on his own bodily history in baptism. I don't deny, of course, that there are forensic and juridical elements in all of this. Certainly if we are united to Christ's body, then God likewise sees us as a part of him, and thus reckons to us Christ's own righteousness through this union. But it is important to respect Paul's bodily language of union and the role this plays in his theology.
Another blog post for another day would address the importance of a real bodily presence of Christ in the Eucharistic elements in light of our bodily union with Christ.