Verse 10-11 : The audience of John wanted to know what exactly they should do in response to his message about repentance. John encouraged them to care for other people’s basic needs (Mark 12:28–31) if they have enough for themselves.
Verse 12: Tax collectors were considered as sinful and immoral people (Mark 2:16; Matthew 5:46) as they collect more money from people by threatening.
John instructed them to collect only what must be taken, and no more. Later in same gospel of Luke, Zacchaeus the tax collector showed repentance by restoring fraudulent wealth to those from whom it was taken (Luke 19:8)
Verse 15-16: John’s sharp messages about repentance and his uncommon lifestyle (Luke 1:80; Matthew 3:4) made the people wonder if he is the Messiah promised by the old testament prophets (Isaiah 9:6–7).
But John refuses to take on the title of Messiah (John 1:19–20) and tells them that the redeemer promised by God is far greater than he is. John is not the Savior; he is a messenger of the Savior (Isaiah 40:3–5; Malachi 3:1; 4:5–6). His calling was to turn Israelites to Jesus. The baptism (immersion) that Jesus gives is from the Holy Spirit (Acts 1:5; 2:38) and fire (Hebrews 12:29).
Verse 17: The wheat represents the believers, whom God will take to heaven and the chaff, unbelievers will be thrown to the fires of hell (Matthew 25:30–31,Malachi 4:1).
Verse 19-20: To have a brother’s wife is against the Law of Moses (Leviticus 18:16; 20:21). Though Herod considered John the Baptist as a prophet (Mark 6:20), his illegal wife Herodias forced Herod for the arrest of John (Mark 6:17–19).