The word "converted" means to change or turn. Specifically, it means to change from one way of life or set of beliefs to another. Sometimes it means "regeneration"—beginning to live a new spiritual life (Psalm 51:10-13, 17). Jesus tells the disciples that their attitudes of ambition are wrong, and they must change or have no part in His Kingdom. To do this, they must be like small children, who, for the most part, lack arrogance and pride. Children are characteristically humble and teachable (I Corinthians 14:20).
According to Mark, Jesus teaches them that, "if anyone desires to be first, he shall be last of all and servant of all" (Mark 9:35). The most humble Christian will be the most distinguished, and he who is willing to be esteemed last and least will be esteemed first. To regard oneself as God regards us is humility. One who receives and loves someone with an innocent child's humble attitude, who may be weak in the faith, displays true Christian character and loves Jesus Christ (Matthew 25:35-40). "Receive" in verse 5 means to approve, love, or treat with kindness; to aid in time of need.
Martin G. Collins
Parables of the Millstone and the Lost Sheep