Verse 10 These questions are in Luke only and the message is addressed to the crowds, that is, the people as such, and not just to their leaders. A life of practical conversion of heart, leading to real service of the neighbour is what John has in mind.
Verse 11 Looking out for the poor is part of Old Testament piety: Is 1:10-20; 58:6-7 and many other texts. At Luke’s stage in the evolution of Christianity, disciples looked forward urgently to a reversal of oppressive social conditions.
Verse 12 As is well known, tax collectors were mostly likely fellow Jews who worked for the Empire and were regarded as traitors and were well known for corrupt practices.
Verse 13 This verse acknowledges the corrupt practices of the tax/toll collectors.
Verse 14 Soldiers, too, could have included Jews, in the service of Herod Antipas. This teaching of John is confirmed in the writings of Josephus.
Verse 15 Some clearly did regard John as the Messiah. He himself sees to have been clear that he was not. However, what he did expect is not so clear: God himself perhaps, or an angel, or the Messiah, or a Moses-type prophet.
Verse 16 John distinguishes himself from the Messiah in three ways. (i) The messiah will be someone “more powerful.” (ii) John uses as a metaphor the humblest task of the lowest servant. (iii) There will be a different kind of baptism. It may well be that originally the image was simpler: wind (pneuma, also spirit) and fire, that is, elements associated with harvest (see the next verse). The Christian reception of the image, however, reads pneuma to mean Spirit, to which the label “holy” is given so that it now refers to the Holy Spirit in baptism. In turn, then, the Holy Spirit has a large presence and role in Luke-Acts (Luke 1:15, 35, 41, 67; 2:25–26; 3:16, 22; 4:1; 10:21; 11:13; 12:10, 12; Acts 1:2, 5, 8, 16; 2:4, 33, 38; 4:8, 25, 31; 5:3, 32; 6:5; 7:51, and so forth).
Verse 17 This is the traditional image of harvest for the end of time. When harvest does come around, it is time to examine the quality of the crop and so it easily becomes a metaphor of judgement, for instance in Psalm 1 above. The image comes from farming practice: the whole mixture was thrown into the air and the wind blew the chaff aside, while the grain landed. The chaff was then burned. Of course, the fire at a harvest was not unquenchable. This points us in the direction of final judgement.
Cf. And they shall go out and look at the dead bodies of the people who have rebelled against me; for their worm shall not die, their fire shall not be quenched, and they shall be an abhorrence to all flesh. (Isaiah 66:24)