Is it acceptable for a born-again Christian to participate in martial arts?
The short answer is yes, a Christian may participate in martial arts, though the issue you raise is broader than just the topic of martial arts.
The Christian is free under the Law of Christ to pursue many things so long as those pursuits are loving toward God and our neighbors, including the principle that we are not a stumbling block to our brothers and sisters (i.e., a cause for sin to other believers).
By the same token, we could also ask about other issues including:
- Can I ride with a group of bikers on a Harley?
- Can I practice meditation?
- Can I visit a bar?
Each of these actions could be the basis for sin, or they could be consistent with holy living. For example, riding a Harley could be an act of rebellion in the heart of a believer, or it could be a recreational activity with the potential to witness to unbelievers within that community. Meditating could be practicing a form of demon worship, or it could be a quiet, prayerful time spent with the Lord through His Spirit. Going to a bar could be a means to drunkenness, or it could be a convenient neighborhood meeting place for fellowship with another believer. And so on.
The question we should ask when evaluating the appropriateness of these activities is: What does my participation in this activity mean to me and to others watching me? In other words, why are you doing it? Motives and witness are everything here.
In the case of your specific issue, there is nothing automatically wrong with participating in martial arts as a sport, but before you decide to participate, ask yourself what your motivation or purpose is in practicing these arts? Is it for exercise or strength or health or self-defense? If so, then you likely have liberty to participate, since your reason for participating is not sinful.
On the other hand, are you practicing martial arts because you have a strong interest in Eastern religion or philosophy? Are you attracted to violence against others and wish to act it out? If so, then martial arts should be avoided, as it is likely to impair your walk with Christ.
Martial arts can be practiced as an Eastern religion, or it can be a valuable sport that helps you to learn one of the fruits of the spirit: self-control. Your motive for participating is the key to finding your answer.
As with all things involving our freedom in Christ, even when we have the right attitude, we must also consider our witness to other believers. Practicing martial arts around a immature believer who is still struggling with past experiences with Eastern religion or teaching would be a poor choice - just as drinking alcohol in front of a recovering alcoholic would be unloving.
Consider both your motives and your potential witness to other believers as you make your choices in these matters.