Well - True. It is true they were broken off; but in order to show that there was no occasion for boasting, he adds that they were not rejected in order to admit others, but because of their unbelief, and that their fate should have a salutary impression on those who had no occasion for boasting, but who might be rejected for the same cause. This is an instance of remarkable tact and delicacy in an argument, admitting the main force of the remark, but giving it a slight change in accordance with the truth, so as to parry its force, and give it a practical bearing on the very point which he wished to enforce.
Thou standest by faith - The continuance of these mercies to you depends on your fidelity. If you are faithful, they will be preserved; if, like the Jews, you become unbelieving and unfruitful, like them you will be also rejected. This fact should repress boasting, and excite to anxiety and caution.
Be not high-minded - Do not be elated in the conception of your privileges, so as to produce vain self-confidence and boasting.
But fear - This fear stands opposed to the spirit of boasting and self-confidence, against which he was exhorting them. It does not mean terror or horror, but it denotes humility, watchfulness, and solicitude to abide in the faith. Do not be haughty and high-minded against the Jew, who has been cast off, but "demean yourself as a humble believer, and one who has need to be continually on his guard, and to fear lest he may fall through unbelief, and be cast off." (Stuart.) We may here learn,
(1) That there is danger lest those who are raised to eminent privileges should become unduly exalted in their own estimation, and despise others.
(2) the tendency of faith is to promote humility and a sense of our dependence on God.
(3) the system of salvation by faith produces that solicitude, and careful guarding, and watchfulness, which is necessary to preserve us from apostasy and ruin.

Well; because of unbelief, etc. - This statement is all true; but then, consider, why is it that they were cast out? Was it not because of their unbelief? And you stand by faith: you were made partakers of these blessings by faith; be not high-minded; let this humble, not exalt you in your own estimation; for if the blessings were received by faith, consequently not by works; and if not by works, you have no merit; and what you have received is through the mere mercy of God. They once stood by faith; they gave place to unbelief, and fell: you stand now by faith; but it is as possible for you to be unfaithful as it was for them, and consequently you may fall under the Divine displeasure, as they have done; be not high-minded, but fear; watch over yourselves with godly jealousy.

Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded, but (t) fear:
(t) See that you stand in awe of God modestly, and carefully.

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.

Well--"Be it so, but remember that"
because of unbelief they were broken off, and thou standest--not as a Gentile, but solely
by faith--But as faith cannot live in those "whose soul is lifted up" (Habakkuk 2:4).
Be not high-minded, but fear-- (Proverbs 28:14; Philippians 2:12):

They were broken off for unbelief, and thou standest by faith - Both conditionally, not absolutely: if absolutely, there might have been room to boast. By faith - The free gift of God, which therefore ought to humble thee.