Most people do not attempt to unify existence. One accepts diverse phenomena as fact.

Many people attempt to unify existence by controlling phenomena and forcing them into apparent unity.

More than a few people attempt to unify existence by deeming the diversity of phenomena irrelevant. Diversity is illusory. This is common mysticism.

Some people try to unify existence by accepting phenomena as phenomena, that is as appearance to a subject. This is a more profound form of mysticism, idealism.

Idealists accept the sense of beyondness embedded in existence – by this stage, beyondness is conceived metaphysically, not in the common exoticized form (which is rooted in an incapacity to see the ordinary world in a spiritual light, and so misunderstands metaphysics to be positing supernatural otherworld populated with supernatural entities) – in a variety of ways.

Some idealists preserve a properly mysterious noumenal being, taken to be the source of phenomena.

Some idealists deny all but phenomena as such. The sense of beyondness is absorbed as an experience. The noumenon is stripped out the content of the sense.

Some try to bracket out, and make irrelevant to discussion, without negation, everything that is not phenomenon. These are the phenomenologists.

Others examine phenomena to see what (if anything) can be known about what is beyond.

Others examine phenomena to see what (if anything) can be known about what is beyond, paying close attention to the forms of knowledge proper to that which seems beyond, versus that which seems immediately intelligible, and that which seems immediately self.

This, in my opinion, is where the gate to religion stands.

[I’ll keep refining this.]