[15] And they brought unto him also infants, that he would touch them: but when his disciples saw it, they rebuked them. [16] But Jesus called them unto him, and said, Suffer little children to come unto me, and forbid them not: for of such is the kingdom of God. [17] Verily I say unto you, Whosoever shall not receive the kingdom of God as a little child shall in no wise enter therein.

Jesus has just finished showing the difference between the Pharisee and the publican in the previous verses. The Pharisee trusted in himself for salvation and the publican trusted in God. Now, Jesus further illustrates his point when the people began to bring young children to Him. He told the people that they had to become like little children in order to “receive the kingdom of God” or be saved. In Matthew 18:3 Jesus explains what he meant. And said verily I say unto you, except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven.
The problem with the Pharisee was that he was not converted. His life centered around himself. A converted person’s life is centered on pleasing God, not himself. All decisions made by an unconverted person are made on the basis of if it pleases themselves. On the other hand the decisions made by a converted person are made on the basis of whether or not it will please God. An unconverted person is proud while a converted person is humble.
This is why in Matthew 18:4 Jesus said whosoever therefore should humble himself as this little child, the same is greatest in the kingdom of heaven. A proud person wants people to think they are “great.” A humble person is only interested in what God thinks. To become like a little child or to be converted, requires a complete change of the mind and can only occur when a person humbles themselves before God, and God works the miracle of conversion by giving them a “new mind”. Philippians 2:5 Let this mind be in you, which was also in Christ Jesus: who being in the form of God, thought it not robbery to be equal with God, but made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men: and being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross. Romans 8:6 For to be carnally minded is death; but to be spiritually minded is life and peace. Romans 7:23-25 But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members. O wretched man that I am, who shall deliver me from the body of this death? I thank God through Jesus Christ our Lord. So then with the mind I myself serve the law of God; but with the flesh the law of sin. 2 Corinthians 8:12 For if there be first a willing mind, it is accepted according to that a man hath, and not according to that he hath not.