A clear and loud answer is, No, Yoga is not at all religious activity. Although unknowingly, people connect Yoga to Hinduism, the reason behind it is that mantras are widely popular in Hinduism.
If you dive one step deeper, you will come to know that Hinduism isn't a religion. Read my article about the Books revealing secrets about Hinduism, visit the link given below to learn more details about it.
Most Importantly, the famous Mantra "OM" is the sound of the Universe; even science validated it.
Just like cardio and other physical exercises, Yoga is an ancient technique of working out with mind and body altogether.
Even the mantras are a collection of the most potent Sanskrit (Ancient Language) words that affect our brain activity positively and enhance our well-being.