God is one in the sense that there is nothing besides Him, both before and after the Creation.
The universe does not make any difference as far as God is concerned.
He remains forever the same without ever changing.
Even though we may be conscious of a world apparently apart from God, this is only the ongoing act of Creation.
The words that made the world are constantly being spoken anew, and nothing can exist unless it is sustained by them.
If they were to cease being spoken, all would revert to emptiness and chaos.
–Rabbi Adin Steinsaltz
From The Long Shorter Way, p. 134, by Rabbi Adin Steinsaltz