Buddhism is a science that developed a following. We could say it’s a science that requires regular practice, but that’s true of so many sciences that it might be redundant to mention regular practice.
Usually a scientist intends to be a static observer rather than one who is necessarily changed by observation. There’s no way to do science of the mind without being changed, though. Being changed is how you gather findings. You are the instrument and the subject. There’s no other way.
When the Buddha was sitting under the tree, he was doing science. This seems clear to me. This is not a matter of claiming Buddhism for science or removing Buddhism from the spirituality section of the bookstore. But it seems essential to understand that a Buddhist is not doing anything very much like what a theist does. A Buddhist is not contacting the beyond. A Buddhist does very immediate science, simply looking and seeing what she can, and seeing if there is a kind of predictability to how the mind works, seeing how much can be understood, and establishing terminology where needed, concepts even, though the goal is to get a better view of how all our existing conceptual baggage operates, and getting some freedom from concepts by seeing how they function amongst our other mental skandhas (“heaps”).
In science, practitioners can share findings, and the findings are broadly applicable. One scientist’s findings are very useful to another scientist. Buddhism is no exception. There is a surprising degree of universality one discovers, a sense in which people are more similar to one another than one had realized, and you can understand other minds by understanding your own.
If someone says something is a religion, then it is. It would be silly, and a very strange impulse, to want to argue that something isn’t a religion if someone says it is. And so Buddhism is a religion to anyone who says it is.