What does Luke 3:7 mean?

John the Baptist was many things: eccentric (Mark 1:6), Spirit-filled (John 1:32–34), and committed to God's will (John 3:30). He was apparently not subtle, nor the type to hold back from speaking truth. His death will come after rightly accusing the local king of having an illegitimate marriage (Luke 3:19–20; Matthew 14:6–10). The people coming to hear him seek to be "baptized," a term transliterated from the Greek word baptizō. This literally means "immersion or submersion," usually with an emphasis on cleaning. Those who made major spiritual decisions or who joined religious groups would often be publicly baptized to demonstrate their commitment.

Here, John speaks directly and clearly to the people who have come to hear his message. The reason for his harsh words was probably the attitude of the people. Some might have come expecting that baptism—the physical act—would somehow make them right with God. John's ministry is about repentance, however. As he explains himself, John will indicate that a person who performs a ritual, without sincere repentance, is subject to God's wrath (Luke 3:8–9). That warning includes encouraging people to live godly, moral lives (Luke 3:10–14).

It's also possible John's rough words are meant to confront local religious leaders and their ardent followers (John 1:19). Matthew's account of John's ministry has these same words being delivered to those men, specifically (Matthew 3:7–12).