There has been much debate over the centuries that have passed since Jesus had this conversation with the religious leader, Nicodemus. The majority of Christian Theologians believe that Jesus, when referring to a man being born of water, was speaking of water baptism. However, I believe that view fails to read the text as it is written. Jesus was teaching us that there are two kinds of births: a water birth which produces a human baby, and a spiritual birth which produces a spiritual "baby." Perhaps the Church has failed to teach new converts that, although the Scripture refers to them as "new men," we all begin as babes in Christ. We need to learn to crawl before we learn to walk in Christ!
The Apostle Peter refers to new converts, saying they need to be fed "baby food" 1 Peter 2:1-3:
"Wherefore laying aside all malice, and all guile, and hypocrisies, and envies, and all evil speakings, as newborn babes, desire the sincere milk of the Word, that ye may grow thereby: If so be ye have tasted that the Lord is gracious."
However, we should not remain "babes" but should grow into mature men and women of God! The Apostle Paul rebuked immature believers in 1 Corinthians 3:1-3, when he wrote:
"And I, brethren, could not speak unto you as unto spiritual, but as unto carnal, even as unto babes in Christ. I have fed you with milk, and not with meat: for hitherto ye were not able to bear it, neither yet now are ye able. For ye are yet carnal: for whereas there is among you envying, and strife, and divisions, are ye not carnal, and walk as men?"
The writer of Hebrews was also critical of immature believers; in 5:12-14, he wrote:
"For when for the time ye ought to be teachers, ye have need that one teach you again which be the first principles of the oracles of God; and are become such as have need of milk, and not of strong meat. For every one that useth milk is unskilful in the Word of Righteousness: for he is a babe. But strong meat belongeth to them that are of full age, even those who by reason of use have their senses exercised to discern both good and evil."
Perhaps the confusion stems from the Apostle Paul's letter in which he uses the phrase "the new man." In Ephesians 4:20-24,he wrote:
"But ye have not so learned Christ; if so be that ye have heard Him, and have been taught by Him, as the truth is in Jesus: that ye put off concerning the former conversation the old man, which is corrupt according to the deceitful lusts; and be renewed in the spirit of your mind; and that ye put on the new man, which after God is created in righteousness and true holiness."
Is it possible that Jesus, in saying, "...Suffer little children to come unto Me, and forbid them not: for of such is the kingdom of God" (Lk. 18:16), was saying that salvation requires one to have child-like faith AND to humbly recognize that as new borne children of God, we need to grow more like Him every day? Should we not continually remind ourselves that we still have a lot to learn? After all, we will not become fully like Jesus until we meet Him face-to-face (1 Jn. 3:2)!