What does Paul mean when he commands the Galatians to “walk by the Spirit”?

The context of Galatians 5 tells us that whatever it means, this practice is opposed to the flesh, our inner tendency to rebellion against God through arrogance and preoccupation with self.

The effects of the flesh are patterns of personal sin; the effects of the Spirit are the virtues of Jesus Christ Himself.

It can be helpful to state what this pneumatic perambulation (walking by the Spirit) is not.

• It cannot be something you find within yourself, like your own spirit apart from God’s influence. “Walk by the Spirit” means God the Holy Spirit.
• It cannot be unassisted intuition or mere sentiment because the desires of the flesh appeal to our affections as their primary point of attack. This is not the “feeling of the Spirit.” Every sin starts with an impulse of the feels.
• Paul is not saying in Platonic fashion to go with your inner spiritual self as opposed to your physical body.

Walking in or by the Spirit is simply stated but profound. This is often the case: a statement’s gravity is often inversely proportional to its size.

πνεύματι περιπατείτε

Bear with me as I try to explain the nuance of two Greek words in modern English:

• πνεύματι = PNEUMATI is the dative singular form of PNEUMA, which means “spirit.” While PNEUMA in Greek can refer to breath, wind, the human spirit, the Holy Spirit, or even worldview, here by context the reference must be to God the Holy Spirit. In this instance the dative case is used instrumentally, suggesting the means by which the verb is accomplished. “By means of the Spirit” would be a literal rendering.
• περιπατείτε = PERIPATEITA is the plural present active imperative form of the verb PERIPATEŌ, which is a common verb in Greek for walking. The present tense on Greek with the imperative mood makes this a command of an ongoing process—“go on walking” or “be walking” as a constant mode of life. Walking is often used in both Old and New Testaments to refer to the mode of one’s life or pattern of conduct.
• What does that give us? “By means of (God) the (Holy) Spirit be walking”

“By means of the Holy Spirit be walking”

There can only be one sense that the believer is responsible to walk by means of the Holy Spirit: dependence on His power. The means by which you walk is to be His strength, His enablement. The means by which one does anything involves the power or enablement to do that thing. This is in direct opposition to walking by means of the flesh. The energy of the flesh is useless to accomplish the will of God.

This arrangement is personal—God the Spirit is the Third Person of the Godhead. But this walk also involves mechanics—He is portrayed impersonally in terms of instrumentation or means. Obviously our volition is involved: we must choose to obey this command to walk. At the same time, the enablement or ability to walk is all from the Lord.

Will you choose to depend upon the power supplied by God the Holy Spirit for the conduct of your life?