Now the works of the flesh are evident, which are: adultery, fornication, uncleanness, lewdness, idolatry, sorcery, hatred, contentions, jealousies, outbursts of wrath, selfish ambitions, dissensions, heresies, envy, murders, drunkenness, revelries, and the like; of which I tell you beforehand, just as I also told you in time past, that those who practice such things will not inherit the kingdom of God. But the fruit of the Spirit is love, joy, peace, longsuffering, kindness, goodness, faithfulness, gentleness, self-control. Against such there is no law. Galatians 5:19-23
Why walk in the Spirit rather than in the flesh? Beside the fact that God commands it, it is a strong evidence that one is either saved or not. None of us walk in the Spirit 24/7 because when we sin, at least in that moment, we are walking in the flesh. Though not sinless this side of Heaven, the redeemed, do not walk predominately in the flesh.
Additionally, the lives of those who walk in the Spirit will differ significantly from those who walk in the flesh. Go back and re-read the description of the fruit of the Spirit.
This fruit will be descriptive of the redeemed. Fruit is the evidence of a union. Where there is no fruit, one might ought to question one’s salvation.
Walking in the Spirit depends on what we do and on what God does. Because God always does His par, when we fail, the fault is ours. When we succeed, all glory belongs to Him.