Jamieson, Fausset and Brown on Hebrews 12:15

15. lest any ... fall--Greek, "lest any (namely, through sloth in running) failing," or "falling short of the grace of God ... trouble you." The image is taken from a company of travellers, one of whom lags behind, and so never reaches the end of the long and laborious journey [Chrysostom].

root of bitterness--not merely a "bitter root," which might possibly bring forth sweet fruits; this, a root whose essence is "bitterness," never could. Paul here refers to De 29:18, "Lest there should be among you a root that beareth gall and wormwood" (compare Ac 8:23). Root of bitterness comprehends every person (compare He 12:16) and every principle of doctrine or practice so radically corrupt as to spread corruption all around. The only safety is in rooting out such a root of bitterness.

many--rather, "the many," that is, the whole congregation. So long as it is hidden under the earth it cannot be remedied, but when it "springs up," it must be dealt with boldly. Still remember the caution (Mt 13:26-30) as to rooting out persons. No such danger can arise in rooting out bad principles.