Religion can be distilled as any belief system that rests explicitly on faith, but if you asked ten theologians what "religion" is, you would get 12 different answers. That is because religion is a complex subject and "religion" is only a word. Like all words, it can mean anything we want it to mean, but in a discussion, it is important that we understand how the word is used.
There is a problem with many of the common definitions of religion. If it has to do with how we relate to God, then Buddhism, Confucianism and core shamanism is left out because they have little to say about God. Many Christians claim that, since religion is a formalized system of knowledge, Christianity is not a religion since it is a relationship with Christ. That contradicts the Biblical author James since he maintained that Christianity is "pure religion and undefiled before God." The Greek word he used was threkeia or "ceremonial observance, worship, religion".