Anytime scripture is talking about the flesh versus the Spirit, be careful. Paul speaking as if to men of flesh, does not mean they are not saved, but that they are not conforming their lives properly, they are living contrary to their true nature of the new man that God created in them, they are living the life of the old man, they are living their life as a lie. If anyone is in Christ they are a new creation, all things have become new and all things are of God.
We are not regard anyone according to the flesh, but understand Paul here is writing about the church of those who will be saved, of those who are saved.
2 Corinthians 5
13 For if we are beside ourselves, it is for God; or if we are of sound mind, it is for you. 14 For the love of Christ compels us, because we judge thus: that if One died for all, then all died; 15 and He died for all, that those who live should live no longer for themselves, but for Him who died for them and rose again.
16 Therefore, from now on, we regard no one according to the flesh. Even though we have known Christ according to the flesh, yet now we know Him thus no longer. 17 Therefore, if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new. 18 Now all things are of God, who has reconciled us to Himself through Jesus Christ, and has given us the ministry of reconciliation, 19 that is, that God was in Christ reconciling the world to Himself, not [d]imputing their trespasses to them, and has committed to us the word of reconciliation.