The mind of the flesh is death, but the mind of the Spirit is life and peace (Rom 8:6).

There are two ways for a human to live:

1) To live as the fulfillment of God’s word

2) To live with his/her own ideas

If these two lifestyles are to be illustrated using the story of Adam and Eve who lived in the Garden of Eve, lifestyle 1) is to eat from the tree of life, and lifestyle 2) is to eat from the tree of knowledge of good and evil. And Paul explains the same two ways of living as follows:

1) To live according to the spirit

2) To live according to the flesh

This is to be understood together with the fact that a human has three parts—spirit, soul and body. A human is essentially a spirit who has a soul and lives in a body. A spirit and a soul are two independent beings, and each has its own will, reason and emotion. God speaks to the sprit, and the spirit speaks to the soul, and the soul controls the body. This is how God’s plan and will are carried out in this physical world, and his word comes to be fulfilled. It is not merely about keeping the law, but about God’s work being done for us through our actions. He always desires to do his will on earth as he does in heaven.

But when our soul stops listening to the spirit, and does not carry out what God says, the fulfillment of his word stops for us. This happens when we live according to our ideas in our soul: that is to say, by eating from the tree of knowledge of good and evil. The result is death—spiritual death, and with no spiritual communication with God, we are left to live with our own fleshly ideas with no spiritual power. In this condition, it is impossible to keep the law, God’s commandments, which are spiritual and requires spiritual power in order for us to keep them.

… the sinful mind is hostile to God. It does not submit to God's law, nor can it do so. Those controlled by the sinful nature cannot please God (vv. 7-8).

So, Christ came to save us from this miserable condition. He lived completely and perfectly according to the spirit. Yet, he was always surrounded by those who lived according to the flesh. Interestingly, the contrast and friction between him and them is seen everywhere in the gospels. We can look at John 6, for example. Some of the crowd whom Jesus fed from five loaves of bread and two fish came to follow Jesus by crossing over the lake. When they found him on the other side of the lake, he said to them

I tell you the truth, you are looking for me, not because you saw miraculous signs but because you ate the loaves and had your fill. Do not work for food that spoils, but for food that endures to eternal life, which the Son of Man will give you. On him God the Father has placed his seal of approval (John 6:26-27).

They wanted to make Jesus the prophet like Moses who had given the manna, or the king-messiah who would give them freedom from the Romans. However, Jesus was to die as the Lamb of God fulfilling the scriptures. While these people approached him attempting to lure him to give them another free meal with their fleshly calculations, Jesus explained to them that he was the true and living bread that came from heaven. Jesus lived in a different world—the reality different from the reality they lived in. They were disappointed with his words, complained and accused him

From this time many of his disciples turned back and no longer followed him (v. 66).