Galatians 5:16 Question

Galatians 5:16 says: “So I say, walk by the Spirit, and you will not gratify the desires of the flesh.”

This would seem to indicate that believers do not always “walk in the Spirit” and when we do not we will “gratify the desires of the flesh.”

Is that the correct way to understand this?

A friend of mine does not think so.

He says: “…believers are not “in the flesh” they are “in the Spirit.”

Thought?