For those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit set their minds on the things of the Spirit. (ESV)
There are many people who do not like the idea of people being different. They will say that there are no good people and bad people. Sin is too strong of a word. People do not sin they just make mistakes. The standard of righteousness in people's minds who think like this has been brought down to their level so that everyone can feel good about themselves. The Scriptures refute such nonsense.
Paul is not afraid to state for us that their are two very different ways that we can live. He sets up for us a very stark contrast. There are those who live according to the flesh and there are those who live according to the Spirit. He would be making a case for those who belong to Satan and those who belong to God. This is the distinction of flesh and Spirit.
Let's consider the one who lives according to the flesh. Paul says that this person sets their mind on the things of the flesh. Before we are saved by God's grace we have one focus and one focus only; feeding our flesh. If it appeals to our sensual appetite we will pursue it. 1 John 2:16 describes our fleshly pursuit as chasing after the desires of the flesh, the desires of the eyes and the pride of life.
The one who has been redeemed by the blood of Jesus Christ has a much different pursuit. Because we are indwelt by the very Spirit of God we now pursue the things that the Holy Spirit wants. We want to know the Savior more and to give him glory. We desire to diminish ourselves and lift up Jesus Christ. This is only possible by the Spirit. The unregenerate man cannot please God.
Do you want to know if you belong to the Lord? Ask yourself what your mind is set on. Things of the flesh? Things of the Spirit? If things of the flesh ask the Lord to forgive your sin and receive his salvation by faith. It is a gift of God's grace.