Paul continued his discussion of the salvation of the Jews.
Romans 11:16-21 (KJV)
For if the firstfruit be holy, the lump is also holy: and if the root be holy, so are the branches.
And if some of the branches be broken off, and thou, being a wild olive tree, wert graffed in among them, and with them partakest of the root and fatness of the olive tree;
Boast not against the branches. But if thou boast, thou bearest not the root, but the root thee.
Thou wilt say then, The branches were broken off, that I might be graffed in.
Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded, but fear:
For if God spared not the natural branches, take heed lest he also spare not thee.
Paul began this section of Romans by alluding to Numbers (verse 16).
Numbers 15:20-21 (KJV)
Ye shall offer up a cake of the first of your dough for an heave offering: as ye do the heave offering of the threshingfloor, so shall ye heave it.
Of the first of your dough ye shall give unto the LORD an heave offering in your generations.
If the first fruit of the dough, the cake, is holy then the entire lump of dough is holy. In the same way, if the root is holy, then so are the branches. What was Paul specifically referring to? Who or what is the root? Who or what are the branches? Let's deal with the second question first.
Paul informed us that some of the branches had been broken off and some wild olive branches had been grafted in (verse 17). These grafted in branches took hold and received nourishment from the root of the olive tree. Paul clearly indicated that the wild olive branches that were grafted in were Gentile believers. Unbelieving Jews must be the natural branches that were broken off.
In verse 18 Paul warned the Gentile believers not to boast against the branches (the Jews). The Gentile believers did not support the root, but the root supported the Gentile believers.
In verse 19 Paul imagined what a Gentile believer might boast about. "The branches were broken off, so that I might be grafted in!" Paul then explained that the branches were indeed broken off because of unbelief, while the Gentile believer is grafted in because of faith (verse 20). However, the Gentile believer should not be full of pride, but should fear, because if God didn't spare the natural branches (the Jews), He might not spare the Gentiles either, if they are found in unbelief (verse 21).
So, the natural branches are the Jews, the natural branches that are broken off are the unbelieving Jews, and the wild olive branches are the believing Gentiles. Who or what is the root? The root is holy according to verse 16. It provides nourishment for the branches (verse 17). Beyond this Romans doesn't necessarily define the root. However, traditionally, the olive tree itself has been viewed as a symbol of Israel. Paul's use of the branches as Jews was consistent with this well-known symbol. The root therefore could refer to the Patriarchs (Abraham, Isaac, and Jacob), Israel, or Yeshua, as many have postulated. In any case, Paul depicted the root as being a Jewish entity. Believing Gentiles are grafted into Israel, the olive tree.
Lastly, let's consider Paul's warning about not boasting against the Jews. Not only historically do we find that the church has treated the Jews horribly, but attitudes today throughout Christianity towards the Jews are less than stellar. Even in situations where Jews aren't ill treated, Christians act like Jews need to be grafted into the church, rather than the other way around. Christians snub their noses at all things Jewish and ignore Torah. But if the Jews were cut off because they strayed from God in unbelief as demonstrated by disobedience, we Gentiles cannot afford to boast against the Jews, or our pride will show that we have strayed into unbelief through disobedience, and will be cut off, as well. This definitely was Paul's wake up call!