The first number that many of us encounter is One. Very young children are taught about quantity using One, Two, Three and so on. Even though good arguments can be made for starting with Zero, it seems that One is the number that we start counting from.
The number One is used to generate all of the positive counting numbers. If you start with Zero and add One repeatedly, you will find them all. Conversely if you multiply any number by One, nothing changes, it remains the same.
The number One wears many guises and is used throughout our language. One speaks of identity, One speaks of completeness, One speaks of unity, One speaks of uniqueness, One speaks of aloneness.
If I have an apple, I have One. If I cut it in half, do I now have Two? Or do I still have One?
Even though the number One is the start of our cardinal numbers (1, 2, 3, …) it is also the end of some mathematical problems. For example, the sum of all probabilities is One.
If you add the fractions 1/2, 1/4, 1/8, 1/16… together you will eventually get ever so close to One. So One can be made up of an infinity of smaller parts. How does it fit them all inside itself?
The three great religions are proudly monotheistic, that is they worship One God. Of course, they do it in slightly different ways with the Christians managing to fit a Trinity into their One God, without apparent contradiction.
Other Eastern faith systems talk about the Universe and everything within it being One (monism). As there are several different ways of describing monism I won’t attempt to discuss it here.
Finally, it is worth having a look at Oneness for a great discussion on how we can experience spiritual Oneness ourselves and in community with others.