Albert Barnes on Romans 11:18

Boast not, etc. The tendency of man is to triumph over one that is fallen and rejected. The danger of pride and boasting on account of privileges is not less in the church than elsewhere. Paul saw that some of the Gentiles might be in danger of exaltation over the fallen Jews, and therefore cautions them against it. The ingrafted shoot, deriving all its vigour and fruitfulness from the stock of another tree, ought not to boast against the branches.

But if thou boast. If thou art so inconsiderate and Wicked, so devoid of humility, and lifted up with pride, as to boast, yet know that there is no occasion for it. If there were occasion for boasting, it would rather be in the root or stock which sustains the branches; least of all can it be in those which were grafted in, having been before wholly unfruitful.

Thou bearest not the root. The source of all your blessings is in the ancient stock. It is clear from this, that the apostle regarded the church as one; and that the Christian economy was only a prolongation of the ancient dispensation. The tree, even with a part of the branches removed, and others ingrafted, retains its identity, and is never regarded as a different tree.

(s) "Boast not against" 1Cor 10:12