A Christian, like all men, is made of three parts (Gen.2:7; 1st Thess.5:23). During his past life, all his thoughts, decisions and feelings were dictated by the flesh. Then he is born again and the spirit is occupied by the Holy Spirit. Now, two sources, the flesh and the spirit call for loyalty, and an ongoing battle ensues within the Christian. Galatians 5:17 says; "For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would". The "babe in Christ" will nearly always lose and the man will continue to obey the dictates of the flesh. But by a diet of good spiritual food, good doctrine and exercise, the Christian starts to mature, and his spirit wins more and more battles. But if the Christian does not mature, his action, even after years, are still CARNAL - that is, having their ORIGIN in the flesh.
He is still a Christian. He is still a child of God. His rebirth is valid and cannot be undone, but he remains "a babe in Christ". And because the flesh is all-pervading, anything that has its ORIGIN in the spirit via the Holy Spirit, is met with the same lack of reaction that you get when you try to explain calculus to a 10 year-old. Paul could not reveal things that had their ORIGIN in the Spirit because they had no capacity, being fleshly, to cope with it.