The term "the One" is misleading (by implication) and the following conclusion that being part of the One is therefore also misleading (by implication).
"Being part of the One" is misleading in the sense that it assumes there is a separate something which is part of some other something.
The problem here is that language is built upon this assumption of "things", thus it's quite difficult to discuss any of this without that assumption infecting the conversation. As example, consider the noun, a fundamental building block of language. The purpose of a noun is to conceptually divide reality. The word "tree" assumes tree is one thing, and soil, water, air, sun are other things. Conceptually it's all very neat and tidy. But in the real world everything is connected to everything else.
Being part of the Wholeness is a physical or mathematical connection, which is common knowledge and has no foundation in spiritualism, but in science.
This is a widely held opinion. There are other widely held opinions. And no proof for any of them.
The word "spiritualism" would seem to refer to spirits, often thought of as some form of intelligence outside of matter, or at least outside of our understanding of matter. It's entirely possible to make a case for something that could be called "spirits" without resorting to any supernatural claims. Not proof of course, just a reasonable case.
What I'm getting at has sometimes been called mysticism. There are many different understandings of that word, but what I mean by it is a focus on experience over ideology. The principle here is that experiences which are sometimes called "spiritual" have value in themselves and explanations are not really required.
As example, if you eat a banana, it doesn't matter what you call that piece of fruit, or whether or not you understand the processes of digestion. The nutrition is in the eating, not in the labeling, analyzing and explaining etc.