One of the ways to preserve an olive tree was to graft in branches from a wild olive tree. The allegory suggests this, which is completely appropriate for ancient practice. The dead branches are destroyed, by Assyria or Babylon. The wild branches are gentiles. Gentile is the definition of anyone who was not of the natural lineage of the house of Israel. This infusion of the gentiles began before the time of Christ when there were a number of gentiles who converted to Judaism, but many more who were sympathizers without officially converting to the religion. As Paul would later explain, one of the hindrances was the Mosaic law’s insistence upon circumcision, which was not only not practiced in the Greek-influenced world, but seen with derision as a defilement of the body. Nevertheless, for the purposes of the allegory, these gentiles added to the number of believers, and they strengthened the faith.
An important lesson for the way God works in our lives is contained in verse 12’s admonition to “go thy way; watch the tree, and nourish it.” We will later learn that this will not be sufficient, but the important aspect is that God will work with us over His time, not ours. While we might hope for a mighty miracle that will change things dramatically and immediately, God moves more slowly. His method is to make small changes and wait for us to learn them and react to them.