John Calvin Commentary Hebrews 12:15

"looking carefully lest [there be] any man that falleth short of the grace of God; lest any root of bitterness springing up trouble [you], and thereby the many be defiled;" — Hebrews 12:15 (ASV)

Looking diligently, or, taking care, or, attentively providing, etc. By these words, he intimates that it is easy to fall away from the grace of God; for it is not without reason that attention is required, because as soon as Satan sees us secure or remiss, he instantly circumvents us. In short, we need striving and vigilance if we are to persevere in the grace of God.

Moreover, under the word grace, he includes our whole vocation. If anyone infers from this that the grace of God is not efficacious unless we ourselves cooperate with it, the argument is frivolous. We know how great the slothfulness of our flesh is; it therefore needs continual incentives. But when the Lord stimulates us by warning and exhortation, He at the same time moves and stirs up our hearts, so that His exhortations may not be in vain or pass away without effect. Then, from precepts and exhortations, we are not to infer what man can do of himself, or what the power of free will is; for doubtless, the attention or diligence which the Apostle requires here is the gift of God.

Lest any root, etc. I do not doubt that he refers to a passage written by Moses in Deuteronomy 29:18; for after having promulgated the Law, Moses exhorted the people to beware lest any root germinating should bear gall and wormwood among them. He afterwards explained what he meant: that is, lest anyone, delighting himself in sin and, like the drunken who are accustomed to excite thirst, stimulating sinful desires, should bring on a contempt of God through the allure of the hope of impunity. This is the same thing the Apostle speaks of now, for he foretells what will take place: that is, if we allow such a root to grow, it will corrupt and defile many. He not only bids everyone to eradicate such a pest from their hearts, but he also forbids them to allow it to grow among them. Indeed, it cannot be otherwise than that these roots will always be found in the Church, for hypocrites and the ungodly are always mixed with the good; but when they spring up, they ought to be cut down, lest by growing they should choke the good seed.

He mentions bitterness for what Moses calls gall and wormwood; but both meant to express a root that is poisonous and deadly. Since, then, it is so fatal an evil, it behooves us to check it with more earnest effort, lest it should rise and creep further.