Romans 11:20

Well
To this the apostle answers, by approving and granting in, part what was said, that the unbelieving Jews were broken off and rejected, and that the Gentiles that believed in Christ were grafted in among the Jews that professed his name; but then he tacitly denies that it was for their sakes, and their account, they were broken off, but for their own incredulity:

because of unbelief they were broken off;
because of their unbelief and contempt of the Messiah, they were rejected of God, and died in their sins; that which excluded their forefathers from the land of Canaan, shut them out of the Gospel church state, and the kingdom of heaven:

and thou standest by faith;
which is not of a man's self, but the gift of God; so that it was not by their merits, and better deservings, but by the grace of God that they were in the situation they were; they were blessed with faith in Christ, and having made a profession of it were admitted to Gospel ordinances, and into a Gospel church; and being helped hitherto to hold the profession of their faith in a becoming manner they stood their ground, and continued in their church relation; and therefore ought not to give way to a vain boasting spirit, but to be humble, modest, and dependent; wherefore he gives them this proper pertinent, and wholesome advice,

be not highminded, but fear.
The apostle would have them not be elated with their gifts, privileges, and enjoyments, and look over others, or down upon them with contempt and disdain, considering that all they had and enjoyed were owing to the goodness of God, and not to any deserts of theirs; and therefore should fear the Lord and his goodness; for not a fear of hell and damnation, or a distrust of the grace of God, is here meant; but a fear of offending him, and that not from a dread of punishment, but from a sense of his grace and goodness; and also designs humility of soul, in opposition to pride, haughtiness, and elation of mind, a lowly carriage and behaviour to others, and an humble dependence on grace and strength from above, to enable to persevere and hold out to the end; for "let him that thinks he stands take heed lest he fall" into sin, ( 1 Corinthians 10:12 ) ; so as to dishonour God and Christ grieve the Holy Spirit, wound his own conscience, and bring himself under the censure of the church, and to be cut off from the good olive tree, the root and fatness of which he now partakes.