How may one refute the religious stonewall argument "all is one"?

[Belacqua]

They say they have had a meditative experience and realized all is one, 

Naturally, the things that people feel when they meditate can't be taken as evidence by others. 

or otherwise postulate that all is one for some other reason. That oneness for no logical reason is extrapolated to be a magical, mystical thing that validates religious views.

This, on the other hand, is very different. 

It depends on the reasons they have for thinking that all is one. Do they assert it for "no logical reason" or do they in fact have reasons? 

I ask because there are a number of different traditions which hold something along those lines. Both Neoplatonism and Buddhism would basically agree with the idea that all is one. And they do so for a number of logical reasons which have been worked out over a long time. 

I'm sure that there are shallow people who repeat "all is one" because they think it sounds cool, or reflects some experience they had while stoned. This doesn't mean that everyone who says it is like that. How you argue would depend on the beliefs of the person you're debating with.

Seems to me there are some seriously fatal flaws in the logic of such a position as "all is one". However I am far from a logician. Could anyone offer some refutations that invalidate such a view?

Before we can offer refutations, we have to know what we're refuting. And if it in fact requires refutation. 

Again, if the people you're talking to just repeat it because it sounds cool, there's not much you can do to debate them. 

But if they know what they're talking about, and have the system of Plotinus, for example, well in mind, then refutation will require a lot more work. You seem to start with the conclusion that they're wrong, but you haven't given us enough information to know whether they really are.