Looking diligently lest any man fail of the grace of God; lest any root of bitterness springing up trouble you, and thereby many be defiled; Looking diligently - Επισκοπουντες· Looking about, over, and upon; being constantly on your guard.
Lest any man fail of the grace of God - Μη τις ὑστερων απο της χαριτος του Θεου· Lest any person should come behind, or fall off from, this grace or Gift of God; this state of salvation, viz. the Gospel system or Christianity; for this is most evidently the meaning of the apostle. It is not the falling from a work of grace in their own souls, but from the Gospel, to apostatize from which they had now many temptations; and to guard them against this, the whole epistle was written.
Lest any root of bitterness springing up - A root of bitterness signifies a poisonous plant. The Hebrews call every species of poison a bitter, and with considerable propriety, as most plants are poisonous in proportion to the quantum of the bitter principle they possess. The root of bitterness is here used metaphorically for a bad man, or a man holding unsound doctrines, and endeavoring to spread them in the Church.
Trouble you - This alludes to the effects of poison taken into the body: the whole animal system is disturbed, sometimes violent retchings, great disturbances through the whole alimentary canal, together with the most fatal changes in the whole sanguineous system, are the consequences of poison taken into the stomach. The blood itself (the principle, under God, of life) becomes putrescent; and probably to this the intelligent apostle alludes when he says, and thereby many be defiled, μιανθωσι, corrupted or contaminated.
Bad example and false teaching have corrupted thousands, and are still making desolation in the world and in the Church.