What does the holy root that supports the branches refer to?

Romans 11:16, Romans 11:18

The image of a holy root in Paul’s example is appropriate for the reality that he wants to describe. To introduce it, the apostle first takes a real example from the temple service and then he exchanges it for a more suitable, albeit fictional one. The example of the dough would not have allowed him to progress easily to concepts such as breaking off’ and grafting in.

Many commentators believe that the examples of firstfruits and root point to one and the same reality, either 1. the patriarchs, 2. the Jewish Christians, or 3. Christ. The Jewish Christians, however, just like the Gentile Christians, are but branches of the same tree. Cranfield¹ admits this and therefore believes that the two images cannot point to the same reality, arguing that the image of the firstfruits points to the Jewish Christians and the image of the root points to the patriarchs. However, since Paul develops the second image in detail, it is plausible that the first, introductory image runs parallel to the second. The remaining choice between patriarchs and Christ essentially boils down to the same thing: for Paul, the patriarchs owe their position to their election by God and to his good pleasure that ultimately culminates in the Person of Christ himself.

The root that supports the branches (old and new) is part of the original tree. As such it is Israel that supports the Gentiles and not vice versa. It would be an error, however, to also identify the root as the Jewish people. Paul even distinguishes the root from the tree as a whole (from which branches are being removed) and this root supports both the circumcised and the (newly entered) uncircumcised. The root can therefore not be identified with the tree; instead, the root must refer to the foundation of believing Israel itself. Hence it directs our thoughts to the promise to Abraham and to the promised Messiah, and the context of Romans 9:1–11:36 also renders it most plausible that the reference is specifically to Jesus Christ himself.²