There is nothing one can do to attain enlightenment. There might be certain impediments that would be beneficial to remove, that's the best it can do.
Removing impediments to enlightenment IS the Buddhist path to enlightenment, right? So in a sense, there is something that people can do. Doing them is the whole point of Buddhist practice, of becoming a monk and so on. (At least that's the case in Theravada, which I'm most familiar with. It might be different in Zen.)
But yeah, nirvana is classified as a/the unconditioned dharma in all the abhidharmas that are still extant. (Theravada and Sarvastivadin.) It isn't something that's brought into being by causal/karmic conditions. So nirvana isn't an object of consciousness that's created or generated by Buddhist practice. Buddhist practice isn't aimed at generating enlightenment, it's aimed at removing what obscures enlightenment.
I think that pretty much all versions of Buddhism would agree on that.
If enlightenment happened to you, you would know it. It's unmistakable. But it's never what the unenlightened imagine it to be.
The Pali suttas certainly seem to emphasize direct experience. People can hear Buddhist enlightenment preached, they can read about it in authoritative scriptures, they can think about it philosophically and logically... but they still aren't going to really know it until they know it firsthand.
The Pali suttas at least tend to speak of nirvana in negative terms, in terms of the elimination of suffering. That suggests that it might be said to be realized when all forms of suffering, and the conditions that bring about suffering, are removed. So it might not be a conventional object of experience in its own right at all. It might not be something that one looks at and recognizes. When all forms of suffering are finally gone, that's it.
That's exactly what Christian fundies say about "salvation".
As I understand it, in Christian salvation is all about getting right with God. It's about restoring a broken interpersonal relationship. There may or may not be an experiential aspect to that, but experience isn't really its central focus, like in Buddhist enlightenment.
But yeah, I think that your comment probably holds true for all forms of religious experience. One doesn't really know it until one knows it.
That seems to leave religious experience fundamentally subjective. It might be totally convincing to the one experiencing the experience. But that person's experience isn't persuasive to other people who aren't sharing the experience. That's going to be as true for Buddhist religious experiences as for any other.