The Lord Jesus tells his disciples that they must become as little children.  But what can this mean?  Many possible ways exist in which to understand this saying.  St. Albert the Great, in his commentary on Matthew 18, 1-5, explains:

" 'Unless you shall become as little children.'  That is, small and innocent, unmindful of injuries, not full of ambition, and not disturbed by lust.  Concerning the first of these characteristics: 'The innocent and the upright shall cling to me' Psalm 24, 21.  Concerning the second: "Let not the sun set on your wrath' (Ephesians 4, 26).  Concerning the third: 'Let us not be made desirous of empty glory, provoking one another, envying one another' (Galatians 5, 26).  Concerning the fourth: 'Do not seek revenge, and remember not the injuries of your fellow citizens' (Leviticus 19, 18).  

" 'You shall not enter the kingdom of heaven.'  That is, the joy of the kingdom.  'You shall not enter': because it is necessary for the one who wishes to 'enter' it to do so with a continuous advance.  'The innocent man shall be saved, but the innocent man of clean hands' (Job 22, 30).  'Who shall climb the mountain of The Lord, or who shall stand in his holy place?  The man whose heart is innocent and who is pure of heart' (Psalm 23, 3-4).  Christ is the Gate of the kingdom.  'I am the gate' (John 10, 9).  Christ is lowly, and the lofty man strikes against the lowly gate.  'No one comes to the Father except through me' (John 14, 6).

" 'Therefore, whoever humbles himself as this little child.'  He here teaches the meaning of humility.  'In malice be children, and in understanding be perfect' (1 Corinthians 14, 20).  'God chooses the foolish of the world to confound the wise' (1 Corinthians 1, 27).  

" 'He shall shall greater in the kingdom of heaven.'  'He who shall be humbled shall be in glory' (Job 22, 29).  'Humiliation follows the proud, but glory upholds the humble of spirit' (Proverbs 29, 23). 'Be humbled under the power of the hand of God, that he may exalt you in the time of his visitation' (1 Peter 5, 6)."

Translating the third of these paragraphs proves a bit tricky because Albert employs a play on words.  The Latin word for "lowly" also means "small", and the word for "lofty", implying "proud", also means "large".  Thus, the large man hits up against the small gate and cannot go through it, and the proud man cannot go "through" the Gate, which is Christ.  The man must become small, first.