"HE WHO IS LEAST IN THE KINGDOM OF HEAVEN IS GREATER THAN JOHN THE BAPTIST"
"I [Jesus] tell you the truth: Among those born of women there has not risen anyone greater than John the Baptist; yet he who is least in the kingdom of heaven is greater than he."
"is greater" = "Positionally greater, not morally. John the Baptist was as great, in strength of character, as any man 'born of woman' but, as to the kingdom, his ministry was to anounce that it was at hand. The kingdom did not then come but was rejected, and John was martyred and the King subsequently crucified. The least in the kingdom, when it is set up in glory... (Lk 1:31-33; 1 Cor 15:24) will be greater than John in the fullness of the Lord's power and glory [as part of the Body of Christ, the Church]. It is not heaven which is in question, but Messiah's earthly kingdom." 1
1C.I. Scofield, (The New Scofield Study Bible, NIV, New York, Oxford University Press, 1967, p.p. 986-987)
Notice that John the Baptist, the greatest of the prophets is still not to be part of the Church of Jesus Christ of which the least of the saints - believers of the Church Age - will be part of and in greater position than John in the Kingdom Age, Christ's Millennial Rule as part of the Body of Christ co-ruling the world over John the Baptist and all the rest of the non-church age saints.
[J. Vernon McGee states, (Thru the Bible with J. Vernon McGee, Vol. IV, Thomas Nelson Publishers, Nashville, 1983, p. 63)]:
" 'Notwithstanding he that is least in the kingdom of heaven is greater than he.' = When the Lord Jesus came, He began calling out a group of people who are even greater than John the Baptist [i.e., the Church, the Body of Christ]. How can they be greater? Because they are in Christ and clothed with His righteousness, [which is unique to this body]."
[Expositors Bible Commentary, NIV, Frank E Gaebelein, Editor, Vol. 8, Zondervan Publishing, Grand Rapids, Mich., 1984, pp. 264-5]:
"In view of the fact that a comparison establishing John as greater than the prophets immediately precedes this text, it is most natural to take ho mikroteros as meaning 'the least' in the kingdom. This entails the view that John the Baptist was not himself in the kingdom...
He was the greatest of the prophets because he pointed most unambiguously to Jesus. Nevertheless even the least in the kingdom is greater yet because, living after the crucial revelatory and eschatological events have occurred... [after the] veiled place in the redemptive history [having unfolded]... he or she points to Jesus still more unambiguously than John the Baptist....
Thus the ground is being laid for the Great Commission: clear witness to Christ before men is not only a requirement of the kingdom (10:32-33) and a command of the resurrected Lord (28:18-20) but the true greatness of the disciple (11:11)."
After John the Baptist we have in view a unique group of believers who are identified with Christ through the indwelling Holy Spirit for the first time in history, the Church who will all be greater in the Kingdom than John the Baptist.