Life in the flesh is the enemy of godliness. It directly opposes God’s will. Rom. 5: 7-5 (GNB) says.
”And so a person becomes an enemy of God, when he is controlled by his human nature; for he does not obey God’s law, and in fact he cannot obey it. Those controlled by their human nature cannot please God. ”
A man is simply said to be living in the flesh not just because he commits sin but when he allows his mind and actions to be controlled by human nature. The easiest way to become an enemy of God is to allow your intellect, your self, your human nature, to control your life. God abhors the old nature no matter the disguise under which he appears: either in blatant sins of disobedience or in display of self life or human wisdom and ability. God counts such a man as an enemy no matter who he is. In John 5:5,6, the LORD Jesus, dealing with Nicodemus answered and said unto him,
”Verily, verily, I say unto thee, except a man be born again, he cannot see the kingdom of God. That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.”
The Lord Jesus concluded in that scripture that, that which is born of flesh is flesh. No matter how much you try to improve it, it can never be anything else but flesh. No matter how much you feed and train a dog, it can never become a goat. Flesh can never be accepted in the kingdom of God no matter how naturally good it is. That which is born of flesh is flesh; and flesh and blood cannot inherit the kingdom of God (1cor. 15:50). It is a matter of birth. Again, in Rom. 8:6-8, the Bible says,
”For to be camally minded is death, but to be spiritually minded is life and peace. Because the carnal mind is enmity against God; for it is not subject to the law of God, neither indeed can be. So, then, they that are in the flesh cannot please God.”
The carnal mind (the self life) leads to death. It does not give peace, neither does it have life in it. Any one who allows himself to be controlled by human nature is only treading the path unto death.
You may have temporary happiness but sincerely, there is no joy inside; it does not give any satisfaction. And actually there is no difference between a carnal Christian and an unbeliever, for their end is the same death.