I don't know any passage that speaks of "walk in the Spirit." No, Romans 8 does not define Christians as those who "walk in the Spirit." You are confusing are in & walk after & making a hybrid "walk in."
There is a difference between "are in the Spirit" and "walk according to (after) the Spirit." Brethren (Christians) who are in the Spirit may nonetheless walk/live after the flesh (be carnal).
There is a difference between "are in the Spirit" and "walk according to (after) the Spirit." Brethren (Christians) who are in the Spirit may nonetheless walk/live after the flesh (be carnal).
I'm not really hung up on the wording, but I don't see that it matters that much.
First of all, the wording of v4 in terms of walking after the Spirit is the same wording used in the manuscripts the KJV relies on for Romans 8:1. So my basic point about the referral of v1 to v4 still stands, and that was the only point I was interested in making to the Onlyists.
Secondly, I don't think Paul makes nearly as clear a distinction as you suggest between those walk after the Spirit and those who are in the Spirit. In fact, the language is suggestive that he simply writes assuming those who are in Christ are also those who are in the Spirit and who walk after the Spirit. The elements can't be extricated.
8 Therefore there is now no condemnation for those who are in Christ Jesus. 2 For the law of the Spirit of life in Christ Jesus has set you free from the law of sin and of death. 3 For what the Law could not do, weak as it was through the flesh, God did: sending His own Son in the likeness of sinful flesh and as an offering for sin, He condemned sin in the flesh, 4 so that the requirement of the Law might be fulfilled in us, who do not walk according to the flesh but according to the Spirit. 5 For those who are according to the flesh set their minds on the things of the flesh, but those who are according to the Spirit, the things of the Spirit. 6 For the mind set on the flesh is death, but the mind set on the Spirit is life and peace, 7 because the mind set on the flesh is hostile toward God; for it does not subject itself to the law of God, for it is not even able to do so, 8 and those who are in the flesh cannot please God.9 However, you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. But if anyone does not have the Spirit of Christ, he does not belong to Him. 10 If Christ is in you, though the body is dead because of sin, yet the spirit is alive because of righteousness. 11 But if the Spirit of Him who raised Jesus from the dead dwells in you, He who raised Christ Jesus from the dead will also give life to your mortal bodies through His Spirit who dwells in you.
But this is beside the main point the thread. Feel free to take it up with me in another thread of PM if you like