How do we know that Christianity is thee religion?

With all the other religions being that there's Jehovah Witnesses, Hinduism, Muslims, Presbyterians and many others religions how can one say that there religion is the one that is real? In christianity If God had not intended for people with distinctive features to intermingle with different people why did this happen if this is what the bible says? The world is filled bi-racial people. How can we learn to coexist if God's intention was to keep the people in groups? Please somebody who knows the christian bible please answer this questions if you can because I'm confused. People from different cultures and backgrounds that worship different Gods are justifying their reasons as to why they think their religion is real and is thee one and it all sounds logical and makes sense which is why I'm so confused.