Hebreos 12:15

Lest any man fail. — Rather, whether any one be falling back from the grace of God. The defection of one member of the community brings loss and danger to the whole body. The last words of Hebreos 10:26 will show what is implied in this “falling back from the grace of God.”

Any root of bitterness. — It is clear that Deuteronomio 29:18, though not formally quoted, is before the writer’s mind. In that chapter Moses had again brought before the people the covenant which, nearly forty years before, had been made and ratified “in Horeb” (see Hebreos 9:18).

With especial solemnity he sets before them the sin and terrible punishment of idolatry, “Lest there should be among you man or woman... 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 marginal note on the last words (“poisonful herb”) explains their true meaning — that which springs from the root is not merely bitter, it is also poisonous.

Again, therefore (see Hebreos 10:27; Hebreos 10:30), the apostasy to which the Hebrew Christians were tempted is compared with the sin committed by those who by idolatry fell away from God’s ancient covenant; and as one idol-worshipper in a community might bring into it a root of bitter poison, so one apostate from the Christian faith would bring trouble and defilement on the Church.

In Hechos 8:23 St. Peter makes reference to the same chapter of Deuteronomy as he speaks to Simon Magus, who, above all other men, proved a root of bitter poison in the early Church.

Many. — Rather, the many (according to the best reading) — i.e., the whole community.