What does Luke 3:17 mean?

When asked if he was the Messiah, John the Baptist always clearly indicated that he was not (John 1:19–20; Luke 3:15–16). In the prior verse, he used a dramatic image to show that the Christ would be much, much greater than he. In an era were the lowliest servants washed the feet of travelers, John the Baptist claims he's not even worthy to untie the Messiah's sandals.

Another part of John's explanation of Christ was the kind of baptism which would be involved. John's baptism was an outward ritual involving water—the Greek term baptizō literally means "immersion." The "baptism" of the Messiah would involve both the Holy Spirit and fire. The combination of that statement, with this verse's emphasis on God separating believers from nonbelievers, leads many to conclude John was speaking of two separate effects of Christ's ministry. Some would respond in faith (Acts 1:5; 2:38), others would respond with rejection and face judgment (2 Peter 3:7; John 3:36).

The metaphor used here involves the process of separating usable grain from inedible husks and stems. The first step is threshing, to knock grain loose from stalks. Next is winnowing. A farmer would use a pitchfork or similar tool to throw threshed grain into the air. This would allow wind to blow the light chaff to the side, while the good grain would fall back into the pile. Inedible or useless material would be collected and burned. Symbolically, this represents God sorting humanity into those who will be saved and those who will be consigned to the fires of hell (Matthew 25:30–31). The same Old Testament works which prophesied John the Baptist's ministry (Malachi 3:1; 4:5–6) make similar remarks (Malachi 4:1).