In the study guide on Galatians, you ask whether our personal experiences of the Holy Spirit have been “sufficient to convince [us] that no particular cultural practices have to be added to what [we’ve] believed about Jesus” (Paul’s Journey Letters, p. 93). Are you using the term “cultural” in a particular, narrow sense? It seems as if everything that we do as humans is in some sense “cultural”—even if it’s simply avoiding “acts of the flesh” such as selfish ambition and drunkenness, or practicing “fruit of the Spirit” such as forbearance and self-control, which Paul mentions at the end of his letter.
You make an excellent point—everything we do is, in some sense, cultural, so if no cultural practices needed to be added to trust in Jesus, then nothing practical at all is expected from those who trust in him, only believing. But as you point out, Paul does expect believers to exhibit a dramatic change in life (not to be accepted by God, but because they have been accepted).
So yes, I am the term “cultural” in a narrower sense. Your question has helped me clarify what this is. What I actually mean by “particular cultural practices” is practices that have been given a religious significance within a particular cultural setting, which people are expected to adopt in order to be recognized and accepted as members in good standing of a religious community. (These are sometimes called “insignia.”)
The main issue in Galatians is whether Gentiles should be required to adopt the practice of circumcision in order to be recognized and accepted as members of the community of Jesus’ followers. As I note in the guide (p. 30), circumcision “has been practiced in a variety of cultures for different ceremonial and medical reasons.” For the Jews it was the necessary sign of community membership. But Paul’s argument in Galatians is that God’s people are now a multinational, multiethnic community whose members are not required to adopt the insignia of any its constituent ethnic or national groups, not even those of the foundational Jewish community (also including sabbath observance, annual festivals, and kosher diet, which he mentions in other letters such as Romans and Colossians).
This would apply equally to the insignia of any modern-day Christian community, such as (for example) not dancing or abstaining from alcoholic beverages. But members of the community of Jesus’ followers everywhere are expected to forsake the “acts of the flesh” and live out the “fruit of the Spirit” as their lives are transformed by the influence of the Holy Spirit and of the community of believers to which they now belong.
See some follow-up thoughts on this topic here.