What is "walking in the Spirit"?

Romans 8:3-8

A while back we talked about Romans 8:3, saying the law was only limited by our inability to keep it. It didn’t fail, but we made it limited. So what the law couldn’t do (save us) God did in Jesus. How? By sending Jesus in real flesh and emptying His wrath towards all sinfulness onto Jesus (Heb 10:1-10).

-V4
Why? So that the law’s requirements (do these things perfectly or die and be judged) would be done away with. Matthew Henry says, “Though the righteousness of the law is not fulfilled by us, yet, blessed be God, it is fulfilled in us…” But this freedom from the law does not automatically apply to everyone. Paul makes a distinction here saying these requirements are done away with for certain people. Who? Those who walk according to the spirit.

-Vs 5-8
Sarx vs Pneuma. These verses expound on the difference between flesh (sarx) and spirit (pneuma) and show the importance of the mind in spiritual matters. Paul is making the claim that sarx brings death while Pneuma brings life. Moo says, “Paul leads to this key claim by tracing people’s manner of life to their underlying way of thinking.” Thinking is central to one’s nature. What you think about largely determines what you do. In verse 6 the word phronema is sometimes translated as mind, but mind-set is better. Phronema means the basic direction of one’s will.
What is your basic inclination? Is your mind oriented towards pleasing God or self? James 3:1 says we all stumble in many ways. There are going to times when we mess up, when we elevate pleasing self above pleasing God. But when we are made new by Jesus there is a change in our basic phronema. New life in the Spirit must involve and start with a change in thinking. It cannot simply be about changing outward actions because then you end up with Pharisees who legalistically force compliance to rules but are not genuinely inclined to please God.
Also, unless your mind-set is changed you will never be able to please God (vs 7- 8). If you try to force behavioral change without changing the direction of the will, you will fail. Someone who’s will and mind are still determined to please self will never be pleasing in God’s sight because at their core they are still sinners. (Illustration: I want my kids to play nicely together. They say please and share toys, all while scowling and being fairly short with each other. I am not pleased even though in the most technical sense they are being nice.) So when we try to force behavioral change but our mind is still not reoriented to pleasing God, our inner self is still hostile towards God regardless of what our actions are, and that is not pleasing to God.

So, here’s where we start getting into the meat of this discussion: what does it mean to “walk according to the Spirit”? The word “walk” in the Bible is often used in a figurative way to mean a general way of life, or to mean trends in behavior. If I rarely do something dangerous, but every once in a while do, you wouldn’t say I walk in danger. But if doing dangerous things is a trend, something I do often, it could be said, “I walk in danger.”
So what does it look like when we shape our general way of life or behavior by the Spirit? Paul gives us explicit insight in Galatians 5:16-25. Go read that passage in Galatians. Here Paul lists identifiers of walking in the flesh and identifiers of walking in the Spirit. Notice the inherent difference between these two lists, other than one is good and one is bad. The first list isn’t just called “flesh” because it’s bad. These are all things you do, actions, outward things. The list of things that are of the Spirit are inner, attitude, thought, well…spiritual. Paul doesn’t contrast sorcery with preaching. He contrasts the acts of the sinful nature against the attitudes of the Spirit.
Paul’s concept of “flesh” isn’t just the acts that are offensive to God. It is the entire system that flesh operates in. So “flesh” means wrong acts, wrong words, wrong desires, AS WELL AS the law’s requirements and penalties for failure. Likewise, acting in the Spirit isn’t just trying to do spiritual things, it’s shifting to the operating system of grace and forgiveness.