Yoga is a set of exercises and meditations that come from Hinduism. Hinduism is a religion that begins in the 16. century BC. The purpose of Yoga is to reach nirvana - the state of bliss, which should be achieved through exercises, meditation and fasting. Many teachers of Yoga instruct their followers to repeat mantra (which is usually in Sanskrit) during meditation. It is most often that people from Europe and USA do not even know what they are saying, and this can be very harmful for them. They also have breathing techniques which they call pranayama. Teachers of Yoga will explain those techniques as something beneficial, but it's just the opposite. One breathing technique is called "Breath of the snake" and  this technique really goes into occult. It is very dangerous to practice such techniques. 

Teachers of Yoga usually belong to the class of Brahmins, and in Hinduism they consider themselves gods. Hinduism is a polytheistic religion. Some of their gods are real historical persons like Krishna, and others are false gods like Brahma, Shiva or Vishnu. They sometimes even consider animals to be gods. This is one of their gods:

I think that the best book that describes Hinduism is "Death of a Guru" by Rabindranath Maharaj, who used to be Hindu priest and a teacher of Yoga, but then converted to Christianity.

Yoga teachers claim that with practicing of Yoga, a person can achieve peace, joy and the state of bliss, but that is impossible without true God, Jesus Christ. The peace cannot be achieved just with exercises and meditation. What brings true peace is a peaceful conscience, and only God can forgive sins. Rabindranath Maharaj describes in his book that he used to practice Yoga for years, but he was always full of anger and aggressiveness. It was when he converted to Christianity, and renounced Hinduism and Yoga, that he achieved true peace and joy.

There are websites, like Christians Practicing Yoga, where they use sentences from the Bible: “Work out your own salvation with fear and trembling; for it is God who is at work in you, enabling you both to will and to work for his good pleasure.”(Philippians 2:12-13). They interpret this sentence as if person has to find his own way to salvation, but with "fear and trembling". Teachings like this are completely wrong. Not only that you don't have to practice Yoga, but you shouldn't practice Yoga at all, exactly because of that passage in the Bible. Jesus never, in the whole New Testament, mentioned that Yoga or Hinduism is a way to salvation. On the contrary, Hinduism is a false religion. Here is a link to a document written by Chris Lawson which is a very good reflection on this problem.

There are people among Christians who are trying to create a syncretism, to join different religions into one. Jesus Christ has revealed us the truth about God, and there is nothing that any other religion could possibly add to his words that are written in the New Testament. The declaration "Dominus Jesus" speaks about this problem.

I want to bring the following passage from the Bible:  "Therefore Jesus said again, "I tell you the truth, I am the gate for the sheep. All who ever came before me were thieves and robbers, but the sheep did not listen to them. I am the gate; whoever enters through me will be saved. He will come in and go out, and find pasture. The thief comes only to steal and kill and destroy; I have come that they may have life, and have it to the full." (John 10,7). This passage refers to all those gurus, avatars, yogis and others that were before Jesus, or that will come in the modern times.

There are many different types of Yoga like: Sahaja Yoga, Ashtanga Yoga, Sivananda Yoga, but all these differences come from different teachers, because Hinduism is practically a conglomerate of beliefs and practices. In USA they have society that promotes Christian Yoga. On their website you can find explanations like this one: "We become more spiritually healthy through the yoga practice by calming our minds and quieting ourselves to the point that we can tune out the world's frequency and tune into God's frequency." To say something like that is absurd because Jesus Christ doesn't have any frequency. What really brings peace is a Christian prayer and meditation. Christian meditation is when you adore before the Blessed Sacrament, when you pray in the Church or at home, when you read Bible or any other Christian book and meditate upon the meaning of what you have read. We all need peace and rest, and Jesus advised his apostles to get some rest. But it is wrong to seek peace in false religions and from false Gods because you are not going to get it.

In Hinduism they also believe in reincarnation, and that is a belief that a man, after his death, reincarnates to another man who lives better or worse, according to his merits or sins from his past life. This teaching is unacceptable in Christianity. God created a man, and in his life he should determine himself for God, and live by his commandments. God gave us Salvation in his Son Jesus Christ, who suffered and died for us on the cross in order to set us free from sin and give us eternal life. This Salvation is a unity with God forever.

Yoga is just a technique, and you can't reach true peace with it, nor can you really find the truth, nor can you achieve salvation because it is a gift of Jesus Christ. All those Chinese and Indians who are saved were saved through Jesus Christ, although they maybe never heard of him.