Yoga is a science- not a religion. It is a science that can work well when practiced along side a religion- any religion. What we call Yoga is Classical Yoga philosophy based on the Yoga Sutras of Patanjali- a very old text that was originally passed down through an oral tradition so dating these scriptures is difficult, but many historians and theologians consider Yoga to be 5000 years old. It is through this text that we learn about the 8 Limbs of Yoga.
1. yamas: restraints or abstinences
a) ahimsa: nonviolence
b) satya: truthfulness
c) asteya: non-stealing
d) brahmacharya: moderation in sensual gratification- sex, eating, sleeping, etc.
e) aparigraha: non greed, non-hoarding
2. niyamas: observances
a) saucha: purity in word, thought and deed, cleanliness
b) santocha: contentment
c) tapas: zeal for practice, discipline
d) svadhyaya: study of spiritual books, self study
e) Isvara pranidhanam: surrender, devotion and/or worship of God
3. asana: physical postures
4. pranayama: control of the breath or life force
5. pratyahara: voluntary sense withdrawal, choosing to turn our focus inward
6. dharana: concentration
7. dhyana: meditation (which is sustained concentration)
8. samadhi: bliss, super conscious state (which is sustained meditation)
The Yamas and Niyamas are similar ethical guidelines as the 10 Commandments of the Bible and they even go further- I love the guideline Santocha- practice contentment. The 8 Limbs to me seem to be in alignment with Christian ideals. We know from the Bible that Christ meditated. The physical postures of Yoga are actually a very small part of the science and are used to make the body and mind healthy enough and calm enough for meditation. Meditation is the cornerstone of Yogic practice. Minds have always been noisy, but the hectic pace of modern life in particular creates a mind that is agitated, stressed and filled with to do lists. Meditation makes the mind quiet enough to hear the voice of Divine guidance- the voice of God- whatever your name is for God. The Yogic science unites the realms of body, mind and spirit. The Yogis learned that breath is the bridge between these realms- so working with the breath is part of the science.
Patanjali used the word "Isvara" for God in the Sutras. There were many other names for God being used at the time which he could have used, but he chose this word specifically. Isvara means "that which rules over the diversity of manifestation". It is close enough to be a Divine blank- so that the Yoga practitioner could use their own personal name for God or even to use no personalized name at all- Classical Yoga may be practiced by people seeing Nature as God or even by atheists.
The name by which we call the Divine is largely based on which house we were born into. The creation story we learned as children is either myth or scripture depending on where or when you were born. God is like the sun in the sky- the sun goes by different names in different languages, but they all describe the same heavenly body.
Patanjali tells us to study spiritual books, but does not say which spiritual books, that is up to us.
Patanjali tells us to surrender to God, but does not tell us which God to surrender to, that is up to us.
To me, this is the genius of the Yogic system- this is why it has been around for 5 thousand years. Many wars have been fought over which name we will call God- many lives have been lost over that fight. Yoga simply takes that element out of the equation so that the practice and science is universal, cross cultural and timeless.
For more information you might read one of Father Bede's many books- A New Vision of Reality: Western Science, Eastern Mysticism and Christian Faith