Paul urged the Galatians to not walk according to their flesh (Galatians 5:16, 25) and told the Corinthian (1 Cor 6:18), Ephesian (Eph 5:3-4) and Colossian believers pretty much the same thing (Col 3:5). In all these cases he was addressing believers. Carnality (or call it whatever you want) is a very real possibility for us.
Positionally, carnal Christians do not and cannot exist. Conditionally, we can.
Rather, Paul is stating facts...not possibilities. YE SHALL NOT. That's a fact. We are not in the flesh...thus we cannot walk according to the flesh. It's absolutely contrary to being in the Spirit.
Gal. 5:16 This I say then, Walk in the Spirit, and ye shall not fulfil the lust of the flesh.
Gal. 5:24-25 And they that are Christ's have crucified the flesh with the affections and lusts. If we live in the Spirit, let us also walk in the Spirit.
And how can that be so? Paul tells us right here. Ye shall not fulfill the lusts of the flesh because the righteousness of the law is fulfilled in us....when we were crucified with Christ. They (the natural man) are after the flesh and mind the things of the flesh...to be carnally minded is DEATH. That certainly isn't any believer.
Romans 8:4 That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit.
Romans 8:5 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.
Romans 8:6 For to be carnally minded is death; but to be spiritually minded is life and peace.