Who doesn't love Eucharist? You get together with all your friends, sip some red wine, and apply yourself to some beautiful painting. You can really feel Christ filling your whole body. You feel so at peace; breathing is easier, and you can focus on your painting and just your painting. The rest of the world goes away, and it's only you, the canvas, and the wine. It's almost like a religious experience, you know? You can totally understand how all those years ago, the apostles totally bought into this. They probably felt the presence of God like, right there with them. And now you can feel it, too! You're now a part of a proud tradition that goes back thousands of years. And you're feeling the benefits of it, just like they did! Yes, after a short, thirty minute session of Eucharist, you leave feeling happier for the rest of the day. You can relax, your relationships seem to improve, and you even feel a little healthier when you get out of bed in the morning. Maybe you'll even put some variations on it. You'll do it while holding a goat in your lap, maybe go all the way and get drunk, who knows? The way you take the Eucharist should be fun for you!
Does this seem like complete hogwash?
Yes? Well that's exactly what you sound like when you conflate meditative stretching/breathing and yoga. Much like you would hesitate to call Wine and Design the Holy Eucharist, you should also hesitate to call a collection of stretches and breathing yoga.
I come from a mixed tradition; I identified as Hindu until the day before my 18th birthday, at which point I began to identify as Christian. Over the course of my life, I've studied various religious traditions in an attempt to learn more about myself and my place in the world. I am by no means an expert on any one religion, but I would still like to highlight some of our (predominantly western laypeople and thinkers) problematic attitudes toward other religious practices. As a result, Hindu readers may be frustrated by my lack of using specific, technical terms. My goal, however, is not to provide a meaning of what yoga is–rather what it is not–in a manner that is approachable to any reader.
Revisiting our Wine and Design analogy, it's fairly easy to see where our perceptions of practices can fall apart when our knowledge is muddy and we fall victim to ignorance. This is not to discredit the experiences you may have during Wine and Design or modern yoga. You may feel more relaxed, or you may feel some kind of spiritual connection. This is the effect of physiological processes like focusing on your breathing and meditating (or even drinking a little bit of alcohol). Modern yoga (as compared to Hatha yoga) is probably what you are familiar with in today's world, and it is not without its benefits. Even I've practiced modern yoga to an extent, and I've felt more relaxed, connected, and at peace. But to say that I'm in any way connected to the expansive experience of a multifaceted cultural practice is a gross oversimplification of what's really going on.
I will not bore you with the details of Hatha yoga, nor will I pretend to speak as an expert on the topic. If you want to learn more, read primary sources. You might read the Vedas (not Wikipedia or random websites!) to gain a more thorough understanding of what we call Hinduism. But even after that (or if you choose not to do that), I recommend creating a solid mental distinction between modern yoga and Hatha yoga, just like you differentiate Wine and Design from the Eucharist. When I conflate these two, I ignore the entire story of Jesus, most of the heroes of the New Testament, and it's hard for anyone to believe that I have an understanding of what's happening in the Bible.
If, I however, said, "I love Wine and Design," without claiming it's associated with the blood of Jesus, that is a far less problematic claim, and it is one with which most will take no issue. And if you want to make the argument that they're the same to you, I'm more than happy to hear you out: just do some homework first.