Except ye be converted στραφῆτε); i.e. turned from proud, ambitious thoughts of worldly dignity. There is no question here about what is popularly known as conversion—the change from habitual sin to holiness.
The conversion here spoken of is confined to a change in the present state of mind—to a new direction given to the thoughts and wishes. The apostles had shown rivalry, jealousy, ambition: they must turn away from such failings, and learn a different lesson.
Become as little children. Christ points to little children as the model to which the members of his kingdom must assimilate themselves. The special attributes of children which he would recommend are humility, unworldliness, simplicity, teachableness,—the direct contraries of self-seeking, worldliness, distrust, conceit.
Ye shall not enter. In the sermon on the mount Christ had said, "Blessed are the poor in spirit: for theirs is the kingdom of heaven" (Matthew 5:3). To all who are not such the gate opens not. That virtue which was unknown to pagan antiquity, the opposite character to which was upholden as the acme of excellence, Christ here asserts to be the only passport to his ideal Church on earth or its eternal development in heaven.
Not the self-esteeming, proud man ( μεγαλο ìψυχος) of Aristotle's worship ('Eth. Nic.,' 4.3), but the humble ( ταπεινο Ìς), the lowly, the self-depreciating, is the man who can realize his position in the spiritual world, and shall be admitted to its blessings and benefits.
St. Paul has summarized the ideal character of the members of the kingdom in 1 Corinthians 13:1-13, especially 1 Corinthians 13:4, 1 Corinthians 13:5, and 1 Corinthians 13:7.