I would define a religion as an organized movement that seeks to bring together a community of people and provide them an explanation for that which reason and natural science cannot explain. It also exists to provide a sense of structure to one’s daily life through rules and guidelines as well as a sense of purpose to that life.
Religion has been around since essentially the beginning of human society. From ancient times to the middle ages up until the modern era, it was often used to justify authority structures governing a society and provide a sense of unity and comradery to its inhabitants. It has allowed and still allows people to make sense of the chaotic and often unjust world that exists around them. Religion has given and continues to give people a sense of hope and an ultimate extra-worldly goal to work towards in their lives through discipline and good habits.
The community aspect, however, is probably the most important factor in making a religion. People with similar beliefs and values are able to come together and exercise those beliefs in values in some way (through congregation, service, church events, etc.). In addition to providing personal goals and standards for oneself, religion allows people to help each other work toward those goals and hold each other accountable. Religion usually allows everyone on board to partake in a tight-knit and welcoming community that further contributes to a sense of purpose.
However, if one allows it, practicing a religion can (but will not necessarily) lead to one closing themselves off from understanding a source of explanation different from their own. A religious community can easily become an echo chamber where no one ever questions the institutional ideas and structure, and one might simply remain in the community for the community’s sake and not because they actually hold all of its values. This, again, is not a necessary factor in an organized religion (it depends on extremity), but it is very possible if people become too reliant on the religion for explanation and decreasingly think for themselves.