I do not think Luke 16:16 and Mark 1:1-4 intimate that John Baptist was " under the gospel dispensation." The kingdom of God might be said to be present in the person and power of Christ (compare Matt. 12:28, and Luke 17:21); but, as far as others were concerned, all that the Lord says on this head, and after John was put in prison, is, "The time is fulfilled, and the kingdom of God is at hand." It was not come for others to enter in till the work of redemption was accomplished, and then it was opened both to Jew and Gentile that believed. "Every man presseth into it," does not imply that any were yet within. It was being preached as nigh both by John and afterward by the disciples; but, whatever the saving mercy of God might have been in past times, and of course then also, it was still an object of search and desire till the cross and resurrection of the Lord. Then it was come, and every believer entered, and the accession of spiritual blessing and privilege was such, that the least in the kingdom was greater than the greatest before, even than John himself, near as he was to it as just at hand. We must bear in mind that, as to John's testimony in John 1:29-34; 3: 29, et seqq., it may have exceeded, more or less, his own intelligence as was often the case in the utterances of the Old Testament prophets. John Baptist did not know more than they, what it is to be purged worshippers, having no more conscience of sins. But this is only one of the many blessings that attach to all within the kingdom now.
I am aware that some, shrinking from the simple meaning because it traverses their preconceived thoughts, have sought to make ó μιχρότερος the least prophet, others (proh, Tudor) Christ himself; but such notions are unwarranted and untenable.