Hear, O Israel: The LORD our God, the LORD is one.Deuteronomy 6:4
Whenever this was first said, it was an important moment in theology because it elucidated a change from worshiping a god to worshiping perfection.
There were many gods, and it wasn't important that any of them be one. But the perfect must be one. I think it is assumed that this is referring to the idea that there is only one God, but this sort of interpretation belies much of the bible- even the commandment not to have other gods before Him- that makes no sense if the people reading, writing, and repeating that didn't think there were other gods out there.
Instead, what is happening is a clear move away from having many gods, and calling them perfect, to having one perfection, and calling it God.
Interestingly, another Jewish tradition, that of not wanting to write or speak the name of God. Then they didn't want to write or speak Adonai, which means Lord. Now, if you happen across a fairly religious Jewish blog, you'll see they type G-d, not even wanting to write a three letter word out. I think the general impression about this is that they hold the name, or whatever referent they are using as so sacred that they don't want to use it, but I believe, at a deeper level, the impulse is to avoid that which would limit. People might make the mistake, as they very often do nowadays, that Israel was worshiping some old man with a white beard, who might look or act like Zeus. They also had a lot of lords in various places who weren't the sort you'd want associated with the Lord, either.
Perfection is a word too, though my hope in using it is that embedded in the concept of perfection is something beyond our understanding or experience.