17, 18. And if--rather, "But if"; that is, "If notwithstanding this consecration of Abraham's race to God.
      some of the branches--The mass of the unbelieving and rejected Israelites are here called "some," not, as before, to meet Jewish prejudice (see on Ro 3:3, and on "not all" in Ro 10:16), but with the opposite view of checking Gentile pride.
      and thou, being a wild olive, wert--"wast"
      grafted in among them--Though it is more usual to graft the superior cutting upon the inferior stem, the opposite method, which is intended here, is not without example.
      and with them partakest--"wast made partaker," along with the branches left, the believing remnant.
      of the root and fatness of the olive tree--the rich grace secured by covenant to the true seed of Abraham.

JFB.