Hebrews 12:15

Looking diligently. No one will drift into heaven. “We must through much tribulation enter into the kingdom of God” (Acts 14:22; see on Matthew 7:21-27; 10:23). We must “strive” to enter in (Luke 13:24). No halfhearted effort will procure the heavenly treasure (see Matthew 13:44-46).

Fail. Greek plessō, “to be lacking in,” “to come short of,” “to fail to reach,” “to be excluded from.” The form of the verb in Greek implies a continuing failure, not a single defection. For comment on falling from grace see on Galatians 5:4.

Root of bitterness. The statement is based on the LXX of Deuteronomy 29:18. In its original setting this was a warning against idolatry (see comment on Deuteronomy 29:18). Here, it seems to be a warning against any querulous person in the church who deliberately sets out to foment ill will and contention among brethren. It is usually a “root of bitterness” growing in the darkness of some shriveled-up soul that blossoms into open and malicious criticism of the appointed leaders of God’s work on earth and that sets brethren at variance with brethren.

Many be defiled. Those whose own souls are bitter always seek to involve as many others as possible in their disaffection and rebellion.