What is religion? That is a question that I have spent most of my life trying to answer. Part of the problem is that meaning depends on context. The answer to the question depends on who is asking whom and why.
My first encounter with the question was when people would ask me if Wicca was a "real religion". even today you will see people answer this question with reference to the U.S. Army Chaplain's Handbook. Which simply begs the question, how did they decide?
The only U.S. Government agency that cares about religion is the IRS. There definition is purely functional. US Code Title 26, 501, c, 3 a corporation is a church if it acts like a church. But they really only care about the money part. They don't define religion.
"(501)(c)(3) Corporations, and any community chest, fund, or foundation, organized and operated exclusively for religious, charitable, scientific, testing for public safety, literary, or educational purposes, or to foster national or international amateur sports competition (but only if no part of its activities involve the provision of athletic facilities or equipment), or for the prevention of cruelty to children or animals, no part of the net earnings of which inures to the benefit of any private shareholder or individual, no substantial part of the activities of which is carrying on propaganda, or otherwise attempting, to influence legislation (except as otherwise provided in subsection (h)), and which does not participate in, or intervene in (including the publishing or distributing of statements), any political campaign on behalf of (or in opposition to) any candidate for public office."
My degree is in Sociology of Religion. You would think that that field would have an agreed definition of Religion but it doesn't. Rodney Stark and William Sims Bainbridge, two leaders in the field, wrote "A Theory of Religion". But it is overly complicated and still riddled with Christian bias. Even the Church, Sect, Cult hypothesis lacks scientific rigor. (Studying religion by classifying religions as Churches, Sects, or Cults is like studying biology by classify people as adults, legitimate children, or bastards.)
The biggest problem with Sociology of religion is that they, like the IRS define a religion as an organization, rather than a system of belief. Organizations are easier to quantify. Defining religion as "what churches do" is like defining food as "what is produced in restaurants". The purpose of food is not the same as the purpose of restaurants. One of the hardest things for me to learn was that churches are businesses that sell religious services. Because they are businesses they have all the same problems or other businesses. That is one of the many reasons that it is important to distinguish between churches and religions. The purpose of any church is maintain it's existence by selling its services. The purpose of all religions is to make meaning for the human mind.
The most practical definition I could find for religion comes from Anthropology. A religion consists of: a Cosmology (a theory of reality the posits Efficacy); a Mythology (a supporting Narrative); a system of Ritual observance (customary Practices); and a Morality (a system of Ethics based on the first three). That is a functional definition. Religion is how human beings make sense of the world around us. We develop a theory that explains why things are the way we experience them. We collect stories that support our theory, and we behave in accordance with our theory.
Regardless of what church you belong to, or don't belong to, your religion is what you believe about the world around you. Everyone has a religion. Your brain can't function without one. If you didn't have beliefs about how the world worked you literally could not get out of bed in the morning.