Rom 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."
When a person trusts Christ as their Savior, it is like a person boarding a plane. Everyone who has a ticket can board the plane but you can either choose to fly economy or first class.
Now, everyone on that flight is going to the same destination and will get there the same time. But on every flight there are some who will enjoy their trip more than others.
Likewise, there are many who have trusted Christ as their Savior but because they are living in the flesh, they are not enjoying their Christian life like those who are living after the Spirit.
“What’s the difference” you ask? It is a difference between life and death! Rom 8:6 "For to be carnally minded is death...” Every thing a carnal believer touches will bring the kiss of death to it! He may be a Sunday School teacher, but if he is a carnal believer, his class will be dead. They may sing specials, but if they are carnal, their singing will only be filled with fleshly emotions but will be void of the Holy Spirit’s power.
On the other hand, Paul says, (v.6b) “but to be spiritually minded is life and peace.” In other words, just the opposite is also true. You take a spirit-controlled teacher and put him in a dead class and he will bring life to it.
One of the greatest hindrances to the cause of Christ is when the world sees a believer living and acting like the world. No wonder Paul goes on to say in (v.8) “So then they that are in the flesh cannot please God.”
I don’t know about you, but I don’t want to settle for being an economy-class Christian, I’m ready to fly first class!