Messiah’s coming will separate grain from chaff. A winnowing fork tossed both into the air. The wind blew the chaff away; the heavier grain fell, to be gathered up from the ground, while the scattered chaff was swept up and burned. The “unquenchable fire” signifies eschatological judgment (cf. Isa 34:10; 66:24; Jer 7:20), i.e, hell (cf. 5:29).
The Baptism of John was pointing towards the great eschatological baptism that Jesus would give to those who trust Him. The promise of the New Covenant is being fulfilled here.