Stretching is not anti-Christian. The spiritual side of yoga is not Christian.
A few years ago, I spent a week with Thich Bhat Hanh, the Zen Buddhist pacifist monk who became so popular when he came to the USA during the Vietnam War.
There was a methodist minister in my sangha group made a very profound statement. He said “ After all my years at the pulpit teaching Christianity, I can honestly say that I never really understood it until now when I look at it from a Buddhist perspective!”
I’ve done that for many years and have found much truth in his statement.