Yes, no and maybe.
Most people in the United States practice a number of disciplines that at one time or another or one place or another had religious overtones, or were a parts of religious practice.
The most obvious is Yoga, which is related to Hinduism, which is a religion practiced by many people in the world. Tai Chi is said to be related in a similar way to Taoism.
In the United States, Yoga is practiced, to a large extent, as a gymnastic exercise to improve health and flexibility and balance. Not as a religion. Some, a very few, practice Yoga (and there are several different schools and divisions of Yoga, from Hatha Yoga through Raja Yoga) as a part of their practice of Hinduism. And good on them all. I’ve known some Hindus, and liked ‘em all.
In Okinawa, karate wasn’t part of a religious observance. It was a way to bust chops, both with and without weapons. And it was practiced in secret because it was, you know, illegal!
Then karate moved uptown, from the poorest prefecture of Japan to the big time in Tokyo. There the names of the kata were changed to make ‘em sound more, you know, Japanese. And the practice of karate was changed to make it seem less, you know, violent. In Shotokan Karate, weapons practice ceased, although that had been an integral part of karate practice in Okinawa (note: General MacArthur didn’t much like a subject population practicing at war games with weapons on his watch, because he wasn’t dumb).
And Karate, along with new names for Kata and a removal of its weapons practice, also got an overlay of Zen Buddhism, kinda like KenDO and JuDO and AikiDO.
Now, a discussion of Zen is way outside the scope of my tiny little blog here. To get a fairly good understanding of Zen, take a look at the wonderful book Zen and Japanese Culture, by D.T. Suzuki.
That overlay of Zen in Karate is invisible to most Americans, and for that matter most Japanese. But all of the Japanese martial arts have it to one degree or another, because they are “do” (the Way) rather than “jitsu” (the technique).
Now, you can practice either Yoga or Karate without any reference whatsoever to any history or religion or background, but knowing it gives you a little more perspective on the way they’re both taught.
And if you ignore the Zen overlay of either Judo or Karate, and you happen by accident to attain enlightenment during your martial arts practice, you’ll probably just take some Rolaids and hope it goes away soon.
And I’m sure it will!
Summary: if you are a Christian or a Jew or a Hindu, you can safely practice karate without much concern about Buddhist Doctrine. Buddhism is, after all, almost more of a therapy than a religion. In general, Buddhism is concerned with suffering and its cause, desire, and the cessation of suffering by managing desire. So Buddhism really doesn’t have a lot of sharp corners to scrape up other religions. And Zen has even fewer, because it take several hundred pages of reading until you feel comfortable that you might have a foggy guess what Zen is even about! On the other hand, I understand that Soto Zen followers believe that seated meditation has great virtue, and Rinzai Zen followers really want to figure out what sort of sound one hand makes when it claps.
When I was much younger, I really, really wanted to find a certified and demanding Buddhist Roshi to teach me by way of the Koans. These days, I think that would be far too much work. Practicing my kata is hard enough!
p.s. many years ago, after an all-day meditation session at a Zen Buddhist Temple, I asked how long until sitting folded up like that didn’t hurt any more. A senior student kindly told me with a smile, “Oh, it always hurts. But after a few years, you don’t care anymore!”
