Looking diligently - This phrase implies close attention. It is implied that there are reasons why we should take special care. Those reasons are found in the propensities of our hearts to evil; in the temptations of the world; in the allurements to apostasy presented by the great adversary of our souls.
Lest any man fail - As every man is in danger, it is his personal duty to see to it that his salvation be secure.
Fail of the grace of God - Margin, “fail from.” The Greek is, “lest any one be wanting or lacking” - ὑστερῶν husterōn. There is no intimation in the words used here that they already had grace and might fall away - whatever might he true about that - but that there was danger that they might be found at last to be deficient in that religion which was necessary to save them. Whether this was to be by losing the religion which they now had, or by the fact that they never had any however near they may have come to it - the apostle does not here intimate, and this passage should not be used in the discussion of the question about failing from grace. It is a proper exhortation to be addressed to any man in the church or out of it, to inquire diligently whether there is not reason to apprehend that when he comes to appear before God he will be found to be wholly destitute of religion.
Lest any root of bitterness springing up - Any bitter root. There is doubtless an allusion here to Deuteronomy 29:18. “Lest there should be among you man, or woman, or family, or tribe, whose heart turneth away this day from the Lord our God, to go and serve the gods of these nations; lest there should be among you a root that beareth gall and wormwood.” The allusion there is to those who were idolaters, and who instead of bearing the fruits of righteousness, and promoting the piety and happiness of the nation, would bear the fruits of idolatry, and spread abroad irreligion and sin. The allusion, in both cases, is to a bitter plant springing up among those that were cultivated for ornament or use, or to a tree bearing bitter and poisonous fruit, among those that produced good fruit. The reference of the apostle is to some person who should produce a similar effect in the church - to one who should inculcate false doctrines; or who should apostatize; or who should lead an unholy life, and thus be the means of corrupting and destroying others. They were to be at especial pains that no such person should start up from among themselves, or be tolerated by them.
Trouble you - By his doctrines and example.
And thereby many be defiled - Led away from the faith and corrupted. One wicked man, and especially one hypocrite in the church, may be the means of destroying many others.