What is religion? If this question asks what all religions have in common, then the answer is: next question, please.
What do all religions have in common? Nothing.
In contrast to Christianity, Islam and Judaism, Buddhism is atheistic with regard to a creator god. There is no doctrine of karma in Christianity. Hinduism is opulently polythesitic, but Islam is not. And so on.
In this kind of situation, it is more promising to offer a simile. What is religion like? Religion is like a cord composed of braided strands (e.g., a rope). The strands overlap and lie over each other in complex ways. The integrity of the cord does not consist in one strand, but in the arrangement of many strands.
Take any family. Look at the faces of its members. Do they have one facial feature in common? No. There are both similarities (e.g., eye color), and differences (e.g., face contour). The relationships are complex, not simple. That is how it is. Just look and see for yourself.
Religion is extremely complex. To make a decent start at understanding it, good questions need to be asked. This is not easy. So, I urge looking first. What is observed? Compare your visual experiences. Look first, ask questions later.
Simon van Rysewyk 