This ecstatic state of mind is cultivated in many religions, probably from the earliest days (not just Indian ones), because the one thing (besides pain) most people want to escape is the one thing that seems to distinguish humans from all other animals — this chattering self-awareness. People escape it with various physical and mental “disciplines” and a wide variety of drugs. That is fine — as a diversion. But it is a pretty useless state to be in, as far as participating in the life of man goes, fulfilling responsibilities, helping the vast loose machinery of this overpopulous species to grind on. In short, it butters no bread and is, paradoxically, quite self-indulgent. It is so because however Mr. A’s self is obliterated TO HIM,and it seems to HIM he is one with everybody, it makes no never-mind to the billions of people around him. He may feel at one with me but I do not therefore feel at one with HIM, which pretty well demonstrates that this ecstatic state is a property of Mr. A and is not a universal reality.

This “oceanic feeling” (and I have been lucky enough to enjoy it, long ago) is just that — a feeling. It happens in a particular spot in a particular brain for a particular time. We do not share it with anybody. If is reflects an external reality, that is as unverifiable as whether what I see and smell is the detection of an external reality. Indeed, it is even less so, since we can use other means to probe the external world and more or less triangulate what reality there might be, independent of our senses. If the reality detected by the oceanic feeling is, say, the quantum sea, or vibrating superstrings (something I once considered) then that is grand, but doesn’t make it holy. It does present a neat question: how come an ability to detect the quantum sea evolved in this primate animal called Homo Sapiens? You can understand how the ability to see, smell, touch, taste, etc. would be very useful to survival and reproduction; but– detecting the vibrations of the superstring continuum, or whatever is actually out there (and in here)? Certainly we have abilities which have no conceivable use in survival — for example, developing higher mathematics and perceiving beauty in the natural world. Apparently these abilities are “unintended consequences” of the structures that underlie and serve more useful abilities, although of course this is mere speculation.
A minor example, of this overdevelopment of some basically useful behavior, which we share with lactating mammals, is the hyperextension of our natural nurturing desires to creatures which are not our progeny: we keep and coddle pets (well, some of us do) and adopt orphan children. Possibly mathematics, like poetry, is the hyperextension of our linguistic/thinking ability, or our very useful ability to detect patterns (which is hyperextended into fallacies of all kinds, including religions — oh, sorry: superstitions).
Well, here were enough points to argue about for now, don’t you think so?

No man is an island, entire of itself; every man is a piece of the continent, a part of the main.
This piece seems visistadvaitic to me: the qualified-non-duality school of thought, which subscribes to one idea that “Self is a part of Brahman”.