When one is born again the flesh becomes more prevalent and so begins the war within between the flesh and the spirit! Paul explains this best of the tug of war within, and how the flesh dies hard when wanting to serve God wholeheartedly (Romans 7:14-25).
Paul goes on to say it is a matter of life and death, for to be carnally minded is death, but to be spiritually minded is life and peace. There is a lot riding on serving God or the old way which one has turned from (Romans 8:5-14). 5 For they that are after the flesh do mind the things of the flesh; but they that are after the Spirit the things of the Spirit.6 For to be carnally minded is death; but to be spiritually minded is life and peace.7 Because the carnal mind is enmity against God: for it is not subject to the law of God, neither indeed can be.8 So then they that are in the flesh cannot please God.9 But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.10 And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness.11 But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you.12 Therefore, brethren, we are debtors, not to the flesh, to live after the flesh.13 For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live.14 For as many as are led by the Spirit of God, they are the sons of God.
Knowing the knowledge of Christ saves us we may wonder does anyone return to the old ways? Sadly some do but scripture teaches it would have been better if they had not known the way of righteousness (2 Peter 2:18-22).