Though a reward had been a thousand times promised to works, yet it is not due but by fulfilling the condition of obeying the law perfectly; and how widely distant are we all from that perfection! Let Papists now go and attempt to force their way into heaven by the merit of works. We cheerfully concur with Paul and with the whole Bible in acknowledging, that we are unable to do anything but by the free grace of God, and yet that the benefits resulting from our works receive the name of a reward.

But let every man prove - That is, try or examine in a proper manner. Let him form a proper estimate of what is due to himself, according to his real character. Let him compare himself with the word of God, and the infallible rule which he has given, and by which we are to be judged in the last great day; compare the Romans 12:3 note; 1-Corinthians 11:28 note; 2-Corinthians 13:5 note.
His own work - What he does. Let him form a fair and impartial estimate of his own character.
And then shall he have rejoicing - That is, he will be appropriately rewarded, and will meet with no disappointment. The man who forms an improper estimate of his own character will be sure to be disappointed. The man who examines himself, and who forms no extravagant expectation in regard to what is due to himself, will be appropriately rewarded, and will be made happy. If, by the careful examination of himself, he finds his life to be virtuous, and his course of conduct pure; if he has done no wrong to others, and if he finds evidence that he is a child of God, then he will have cause of rejoicing.
In himself alone - Compare Proverbs 14:14; "A good man shall be satisfied from himself." The sentiment is, that he will find in himself a source of pure joy. He will not be dependent on the applause of others for happiness. In an approving conscience; in the evidence of the favor of God; in an honest effort to lead a pure and holy life, he will have happiness. The source of his joys will be within; and he will not be dependent, as the man of ambition, and the man who thinks of himself more highly than he ought, will, on the favors of a capricious multitude, and on the breath of popular applause.
And not in another - He will not be dependent on others for happiness. Here is the true secret of happiness. It consists:
(1) In not forming an improper estimate of ourselves; in knowing just what we are, and what is due to us; in not thinking ourselves to be something, when we are nothing.
(2) in leading such a life that it may be examined to the core, that we may know exactly what we are without being distressed or pained. That is, in having a good conscience, and in the honest and faithful discharge of our duty to God and man.
(3) in not being dependent on the fickle applause of the world for our comfort. The man who has no internal resources, and who has no approving conscience; who is happy only when others smile, and miserable when they frown, is a man who can have no security for enjoyment. The man who has a good conscience, and who enjoys the favor of God, and the hope of heaven, carries with him the source of perpetual joy. He cannot be deprived of it. His purse may be taken, and his house robbed, but the highwayman cannot rob him of his comforts. He carries with him an unfailing source of happiness when abroad, and the same source of happiness abides with him at home; he bears it into society, and it remains with him in solitude; it is his companion when in health, and when surrounded by his friends, and it is no less his companion when his friends leave him, and when he lies upon a bed of death.

Prove his own work - Let him examine himself and his conduct by the words and example of Christ; and if he find that they bear this touchstone, then he shall have rejoicing in himself alone, feeling that he resembles his Lord and Master, and not in another - not derive his consolation from comparing himself with another who may be weaker, or less instructed than himself. The only rule for a Christian is the word of Christ; the only pattern for his imitation is the example of Christ. He should not compare himself with others; they are not his standard. Christ hath left us an example that we should follow his steps.

But let every man prove his own work,.... Not concern himself about the actions and works of others; let him review his own heart and actions; let him examine, try, and prove his whole conduct in life by the rule of God's word, when he will find enough at home, without bearing hard upon, and censuring others:
and then shall he have rejoicing in himself alone, and not in another; which is either ironically said, he will then see what reason he has to rejoice and glory in his own works, and vaunt over others, and to boast of his performances, and despise others; so far from it, that he will have reason to be ashamed of himself, and to own and acknowledge his unworthiness and unprofitableness: or if, upon such a review, examination, and probation of his works, it shall appear that he has had his conversation in the world, by the grace of God, in simplicity and godly sincerity, this testimony of his conscience will be his rejoicing; see 2-Corinthians 1:12. He may rejoice "in himself", in his own works, as the fruits of grace, but not as the effects of his own power and strength; and may glory and boast of them before men, in vindication of his cause and character, and as evidences of the truth of grace, but not before God, as if they were the matter of his justification and acceptance:
and not in another; that is fallen into sin; making use of his sins and faults to set off himself, and to increase his own praise and condemnation; rejoicing in this, that he is better than others, and is not, as the Pharisee said, as other men are, as wicked as they, or has not fallen into such sins as others have done. He will have occasion to take such a method as this, if his conversation will bear the test; he will have rejoicing in the testimony of his own conscience, and will have no need to compare himself with others; his glorying will be on account of his own actions, and not through a comparison of other men's. This no ways contradicts a man's glorying in God, and rejoicing in Christ Jesus alone, in the business of salvation. It only regards a man's glorying before men, in a modest and humble manner, of what he is enabled to do, by the grace of God, without fetching in the characters of other men that are wicked, or have fallen, to illustrate his own.

his own work--not merely his own opinion of himself.
have rejoicing in himself alone--Translate, "Have his (matter for) glorying in regard to himself alone, and not in regard to another (namely, not in regard to his neighbor, by comparing himself with whom, he has fancied he has matter for boasting as that neighbor's superior)." Not that really a man by looking to "himself alone" is likely to find cause for glorying in himself. Nay, in Galatians 6:5, he speaks of a "burden" or load, not of matter for glorying, as what really belongs to each man. But he refers to the idea those whom he censures had of themselves: they thought they had cause for "glorying" in themselves, but it all arose from unjust self-conceited comparison of themselves with others, instead of looking at home. The only true glorying, if glorying it is to be called, is in the testimony of a good conscience, glorying in the cross of Christ.

But let every man try his own work - Narrowly examine all he is, and all he doeth. And then he shall have rejoicing in himself - He will find in himself matter of rejoicing, if his works are right before God. And not in another - Not in glorying over others.