matthew 3:12 Whose fan is in his hand, and he will throughly purge his floor, and gather his wheat into the garner; but he will burn up the chaff with unquenchable fire. - Free Bible Online

      12. Whose fan--winnowing fan.
      is in his hand--ready for use. This is no other than the preaching of the Gospel, even now beginning, the effect of which would be to separate the solid from the spiritually worthless, as wheat, by the winnowing fan, from the chaff. (Compare the similar representation in Mal 3:1-3).
      and he will throughly purge his floor--threshing-floor; that is, the visible Church.
      and gather his wheat--His true-hearted saints; so called for their solid worth (compare Am 9:9; Lu 22:31).
      into the garner--"the kingdom of their Father," as this "garner" or "barn" is beautifully explained by our Lord in the parable of the wheat and the tares (Mt 13:30, 43).
      but he will burn up the chaff--empty, worthless professors of religion, void of all solid religious principle and character (see Ps 1:4).
      with unquenchable fire--Singular is the strength of this apparent contradiction of figures:--to be burnt up, but with a fire that is unquenchable; the one expressing the utter destruction of all that constitutes one's true life, the other the continued consciousness of existence in that awful condition.

      Luke adds the following important particulars (Lu 3:18-20):

Lu 3:18:
      And many other things in his exhortation preached he unto the people--showing that we have here but an abstract of his teaching. Besides what we read in Joh 1:29, 33, 34; 3:27-36, the incidental allusion to his having taught his disciples to pray (Lu 11:1) --of which not a word is said elsewhere--shows how varied his teaching was.

Lu 3:19:
      But Herod the tetrarch, being reproved by him for Herodias his brother Philip's wife, and for all the evils which Herod had done--In this last clause we have an important fact, here only mentioned, showing how thoroughgoing was the fidelity of the Baptist to his royal hearer, and how strong must have been the workings of conscience in that slave of passion when, notwithstanding such plainness, he "did many things, and heard John gladly" (Mr 6:20).

Lu 3:20:
      Added yet this above all, that he shut up John in prison--This imprisonment of John, however, did not take place for some time after this; and it is here recorded merely because the Evangelist did not intend to recur to his history till he had occasion to relate the message which he sent to Christ from his prison at Machærus (Lu 7:18, &c.).

JFB.