In simple terms, convert also means "to change," as in ice to water or dollars to pesos. Theologically, it means changing from sinner to saint, filthy to holy, worldly to godly. In Acts 3:19, Peter uses "repent" and "convert" together. Both entail a recognition of self and sin and beating a hasty path to righteousness. Paul explains the repentance, conversion, and salvation process by contrasting two terms. We must not be conformed to the world ("similar to, identical to, in agreement with, or compliant"), but transformed ("changed in composition or structure, character, or condition, converted"). Repentance means changing one's whole life!
Martin G. Collins
Basic Doctrines: Repentance
Suppose we were asked, "Who are the greatest people in your town?" What would we turn to as qualities that exemplify greatness? Would it be money, prestige, learning, or military conquest? Just before this incident, the disciples were arguing about which of them would be greatest among them (Mark 9:33-37). Jesus responds to the disciple's question by using a child.
He is not saying that heaven is populated by little children. He means that a child's attitudes of easy dependence, trust, unpretentiousness, awareness of weakness, lack of knowledge, and submission to parents illustrate what we as converted adults must become toward God. Jesus is not saying every child is like His illustration; He is using one to illustrate an ideal. It is essential that we grasp that we must turn, change, to become like the ideal.
John W. Ritenbaugh
Fully Accepting God's Sovereignty, Part Three: The Fruits
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
Other Forerunner Commentary entries containing Matthew 18:3: