When a person realizes that (his) personal consciousness and the cosmic consciousness are the same, he realizes that "I am the Infinite Reality" or "I am the Ultimate". He can't see anything other than he himself. It is 'something' without any parts. It is eternal, unchanging, present at all times, immutable, undivided and infinite. By that statement if you mean the changing universe as you see now, you can't say that ultimately...for it is unchanging.
Since this is the truth, when you realize the truth you realize that this was true even before you realize it. So, before you realize the truth you can say "I am the ultimate". And it must be so. But since you are unaware of "I", even you know that that statement is only a sheer folly. Then what about others?
I believe, that statement is not to be shouted out for gaining something. It is to be experienced/realized it as your SELF.
When you experience this, you realize who others and other things are. And it will be a knew knowledge to you.
I understand that we are all part of the universe. But how AM I the
Since the person 'sees' the same soul everywhere, he sees nothing to use a plural form.
How do people come to that conclusion?
All people can't come to that conclusion in their present life.