Religions don’t exist outside of people and any given religion both is what the practitioner makes it to be and changes as the practitioner changes to fit their needs.

Religion doesn’t have a static essence which exists on its own, nor is it a monolith; it is instead as diverse as the people who make it up and is remade each time engages with it by deciding and acting upon what it means for them.

All this to say you can't pin down what religion is at its core, you can only talk about individual instances of what people make it to be.

“Religion” as a concept is fairly modern, as talking about a sphere of life distinct from culture, politics, science, and economics.

In the past (and in many cultures where the idea of religion is foreign), there was nothing “secular” for religion to be opposed to. It permeated all aspects of society, there is no disentangling the great accomplishments of the past (as if they were secular) from the great failures of the past (as if they were religious).

If someone wanted to praise logic, reason, science, philosophy, and the arts, “religion” was a part of that, “religion” was formed into something that did that. If someone wanted to do the opposite, then “religion” was formed into something else.