Ever since Eden, man’s own doctrine has been unsound doctrine: the serpent deceived Eve, Adam listened to his wife instead of God, and the ground was cursed as a result (Genesis 3:13-18). Sound doctrine instead centers on Christ, the Word who became flesh and dwelt among us, the one who spoke only what the Father gave Him to say and whose commandment is eternal life (John 1:14; John 12:49-50).
Scripture itself is the God-breathed standard for that doctrine, useful for teaching, reproof, correction, and training so that the man of God may be complete (2 Timothy 3:16-17), and Titus 2 sketches what sound doctrine looks like lived out across a whole congregation, young and old, men and women. Because a time is coming when people will not endure sound teaching but gather teachers to suit their own passions (2 Timothy 4:3-4), the sermon closes by returning to the Shema’s call to teach these truths diligently to the next generation (Deuteronomy 6:4-9).