Jamieson, Fausset and Brown on Luke 12:49-53

49-53. to send--cast.

fire--"the higher spiritual element of life which Jesus came to introduce into this earth (compare Mt 3:11), with reference to its mighty effects in quickening all that is akin to it and destroying all that is opposed. To cause this element of life to take up its abode on earth, and wholly to pervade human hearts with its warmth, was the lofty destiny of the Redeemer" [Olshausen: so Calvin, Stier, Alford, &c.].

what will I, &c.--an obscure expression, uttered under deep and half-smothered emotion. In its general import all are agreed; but the nearest to the precise meaning seems to be, "And what should I have to desire if it were once already kindled?" [Bengel and Bloomfield].