Why would Paul exhort “unbelievers” to walk in the Spirit?
Paul is teaching the church at Galatia and us that we must walk according to the Spirit, rather than walking according to the flesh.
- I say then: Walk in the Spirit, and you shall not fulfill the lust of the flesh.
The same Christians whom Paul exhorted to walk according to the Spirit is the same Christians that he warned not to walk according to the flesh and thereby practicing the works of the flesh.
He plainly says that the Spirit is against the flesh. Referring to people with the Holy Spirit in them.
For the flesh lusts against the Spirit, and the Spirit against the flesh; and these are contrary to one another,
so that you do not do the things that you wish.
I say then: Walk in the Spirit, and you shall not fulfill the lust of the flesh. For the flesh lusts against the Spirit, and the Spirit against the flesh; and these are contrary to one another, so that you do not do the things that you wish. But if you are led by the Spirit, you are not under the law.
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. Galatians 5:16-21
He taught the church in Rome and Corinth and Ephesus the same thing.