The sermon explores the theological doctrines of justification, imputation, and adoption through the narrative of Philemon and Onesimus. It explains that justification involves Christ paying the sinner's debt in the divine court, while imputation transfers Christ's righteousness to the believer's account. Furthermore, adoption signifies being welcomed into God's family as a beloved child. The preacher emphasizes that these truths provide believers with victory over sin and assurance of salvation through faith in Jesus Christ.