Charles Spurgeon Commentary Hebrews 12:15-16

"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; lest [there be] any fornication, or profane person, as Esau, who for one mess of meat sold his own birthright." — Hebrews 12:15-16 (ASV)

Lest any root of bitterness springing up trouble you, and thereby many be defiled; lest there be any fornicator,

Fornication was the special sin of that age: in fact, so common was it that the heathen did not consider it to be a sin at all. Knowing of the tendency to licentiousness in all around them, Paul specially warned the Hebrew Christians against that horrible evil.