Thomas Brooks emphasizes the grave danger of bosom sins, which are cherished sins that individuals hold onto despite their destructive nature. He contrasts the attitudes of unsound hearts, which cling to these sins as if they were precious, with the resolve of sound Christians who recognize these sins as the most heinous and detrimental to their relationship with God. Brooks argues that a sincere believer must actively fight against these sins, viewing them as more hideous than the devil or hell itself, and understands that they are the greatest obstacles to spiritual comfort and communion with God. The sermon calls for deep introspection and a commitment to repentance and vigilance against these 'darling sins'.

What bosom sin is there so sweet or profitable that is worth a-burning in hell for, or worth a-shutting out of heaven for? Surely none! This a gracious soul seriously weighs, and accordingly he sets himself against the toad in his bosom—against his darling sins, against his complexion sins. But now, unsound hearts are very favorable to bosom sins, to complexion sins. They say of them, as Lot of Zoar, "Is it not a little one?" Gen. 19:20; and as David once said concerning Absalom, 2 Sam. 18:5, "Deal gently with the young man." An unsound heart is as fond of his bosom sins, of his darling sins— as Jacob was of his Benjamin; or as Naaman was of his idol Rimmon; or as Judas was of his money-bag; or as Herod was of his Herodias; or as Demetrius was of his Diana. The besotted sinner is most engaged to his bosom sins, his darling sins; and therefore it is as bitter a thing as death for him to part with them. He had rather part with thousands of rams, and with ten thousand rivers of oil; than with his bosom sin. Let God frown or smile, stroke or strike, lift up or cast down, promise or threaten—yet he will hide and hold fast his bosom sin! Let God set life and death, heaven and hell, glory and misery before him—yet he will not part with his bosom sins! Let God wound his conscience, blow upon his estate, leave a blot upon his name, crack his credit, afflict his body, write death upon his relations, and be a terror to his soul—yet will he not let his darling sins go! An unsound heart will rather let God go, and Christ go, and heaven go, and all go—than he will let his darling lusts go! But now a sound Christian, a thorough Christian, he sets himself most against the Delilah in his bosom, against the Benjamin, the son, the sin of his right hand. A sincere Christian looks upon bosom sins, upon complexion sins, as the most God-provoking sins. There are no sins so provoking to God's jealousies and justice as bosom sins! He looks upon bosom sins as the most dangerous sins! He looks upon bosom sins as the worst thing in all the world! He looks upon bosom sins as more ugly and horrid than the devil himself, or than hell itself! He looks upon bosom sins as the great hindrance between God and his soul, and between his conscience and his comfort. He looks upon bosom sins as those enemies that have provoked God often to turn a deaf ear to all his prayers! He looks upon his bosom sins as so many Judases that have often betrayed him into the hands of the devil! He looks upon his bosom sins as the waters of Marah, which have embittered all his mercies! He looks upon his bosom sins as the only things that have often clouded the face of God! He looks upon his bosom sins as dead flies in the box of precious ointment—which spoils all; and accordingly with all his might he sets himself against them. He fights most against these; he weeps most over these; he watches and arms most against these; he prays most against these; he resolves most against these; he lays the axe of repentance most to these.