But if some of the branches were broken off, and you, although a wild olive shoot, were grafted in among the others and now share in the nourishing root of the olive tree, do not be arrogant toward the branches. If you are, remember it is not you who support the root, but the root that supports you. Then you will say, “Branches were broken off so that I might be grafted in.” That is true. They were broken off because of their unbelief, but you stand fast through faith. So do not become proud, but fear. For if God did not spare the natural branches, neither will he spare you. Note then the kindness and the severity of God: severity toward those who have fallen, but God's kindness to you, provided you continue in his kindness. Otherwise you too will be cut off. And even they, if they do not continue in their unbelief, will be grafted in, for God has the power to graft them in again. For if you were cut from what is by nature a wild olive tree, and grafted, contrary to nature, into a cultivated olive tree, how much more will these, the natural branches, be grafted back into their own olive tree.
When I looked at what Schreiner wrote about this passage, I wasn’t surprised that he dealt with the warning better than N. T. Wright did.
I was also pleasantly surprised that the compared the boasting being warned against here with the boasting Paul refutes and condemns in Romans 1-4. He is right to do so, but it felt like I was reading a different book by a different author.
In his comments on Romans 1-4, Schreiner seems quite intent of proving that the “boasting” is more about bragging in one’s moral accomplishments rather than one graciously-given standing with God.