Paul says if you are 'in the flesh' you are not born again. He qualifies being 'in the flesh' as meaning 'without the Spirit'.
"you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you"-Romans 8:9
Not to be confused with the born again Christian who is being 'fleshly':
"for you are still fleshly. For since there is jealousy and strife among you, are you not fleshly, and are you not walking like mere men?"-1 Corinthians 3:3
Christians, by virtue of possessing the Holy Spirit in salvation, can never be 'in the flesh', but they can be 'fleshly', still being immature infants in Christ.
This is why I have stopped using the phrase 'in the flesh' to refer to born again believers who are acting in the power and guidance of the flesh and not in the power and guidance of the Spirit. 'In the flesh' does not mean that. Being 'in the flesh' means you are not born again.