It's kind of a loaded question (though not intentionally I'm sure!
) because "religious" means so many different things to different people and in different contexts (thus my choice of "other"). I do think that humans are naturally spiritual, as an extension of our nature as reflective beings. Even the choice to be non-spiritual is a spiritual choice, and I suspect that if we truly weren't spiritual beings the question just wouldn't make any sense to us.
To clarify, by "spiritual beings" I do not necessarily mean that we have a "spirit" aspect (though I personally think we do, sort of), but that it is part of our nature to ponder our place in the grand scheme of things--or for that matter to even grasp that there
is a grand scheme of things--rather than to just act to meet our needs and wants, with the scope of our reflection limited to our own actions and experiences.
I think this is a very different thing than an explanation for religion and/or spirituality that I have sometimes heard from non-religious or anti-religious people, that people need (or think they need) religion to explain the workings of the universe or its origins. While religion has often served that purpose, it is my opinion that that has never been its primary function in people's lives.