Christians speak against yoga because its origins denote it as part of ancient east Indian religious practices. Here's what the country who holds the origins of yoga say on the matter.
According to
https://www.mea.gov.in/in-focus-article.htm?25096/Yoga+Its+Origin+History+and+Development an official government site of India...
"Yoga is essentially a
spiritual discipline"
"As per Yogic scriptures the practice of Yoga leads to the union of individual consciousness with that of the Universal Consciousness"
"Thus the aim of Yoga is Self-realization"
"Yoga has proved itself catering to both material and
spiritual upliftment of humanity"
According to
http://www.yogabasics.com/learn/history-of-yoga/
"The word yoga was first mentioned in the oldest
sacred texts, the Rig Veda. The Vedas were a collection of texts containing songs, mantras and rituals to be used by Brahmans, the
Vedic priests."
"Yoga was slowly refined and developed by the Brahmans and Rishis (
mystic seers) who documented their practices and beliefs in the Upanishads, a huge work containing over 200
scriptures."
"The most renowned of the Yogic
scriptures is the
Bhagavad-Gîtâ (Hindu scripture), composed around 500 B.C.E"
"In the late 1800s and early 1900s, yoga masters began to travel to the West, attracting attention and followers. This began at the 1893
Parliament of Religions in Chicago, when Swami Vivekananda wowed the attendees with his lectures on
yoga and the universality of the world’s religions "
According to
https://yogasix.com/history-of-yoga/
"however, the physical exercise is a part of the multidimensional practice; it has a meditative and
spiritual core."
"One of the six major orthodox schools of
Hinduism is also called
Yoga, which has its own epistemology and metaphysics "
According to
https://www.allaboutspirituality.org/history-of-yoga.htm
"the practice of yoga is much more than a system of physical exercise for health. Yoga is an ancient path to spiritual growth"
"Since it is taught by the yogis that everything is God, it then stands to reason, man is God"
"
Yoga views man’s problem primarily in terms of ignorance.
Man simply does not understand he is God so the solution is enlightenment, or an experience of union with God."
"
Can the methods used in the yoga techniques for exercising be separated from the philosophy? The answer is a distinctive “
no” because yoga is considered to be a practice of psychosomatic exercises."
So the foundation of yoga is in 5000 year old pre-Hinduism and declares it a spiritual act with the intent to realize you are God to obtain universal consciousness. I don't need to try it myself to read the history, roots, origins and purpose to know what the intention of it has been for 1000's of years. That act of doing yoga, actually, in no way qualifies anyone to know whether it qualifies as evil or not. So that is a false logic. By that logic i don't know murder is evil because i've never killed anyone. But that's not the way the world operates.