Chandogya Upanishad is strong in Advaita (non-duality). One turning into many is Advaita. But I have not checked if it mentions Brahman as God. I will check.
"What is the origin of this world? Space, said he.
Verily, all things here arise out of space. They disappear back into space, for space alone is greater than these, space is the final goal." Chandogya Upanishad 1.9.1
"This is my Self in the innermost heart, greater than the earth, greater than the aerial space, greater than these worlds. This Self, this Self of mine is that Brahman." Chandogya Upanishad 3.14.3
— Chandogya Upanishad, 6.8 - 6.16
A father's (Uddalaka Aruni) advice to the son (Svetaketu):
"Translation 1: This universe consists of what that finest essence is, it is the real, it is the Self, that thou art, O Śvetaketu!
Translation 2: That which is the finest essence – this whole world has that as its Self. That is Reality. That is Atman (Self). That art thou, Śvetaketu.
Translation 3: That which is this finest essence, that the whole world has as its self. That is the truth. That is the self. In that way are you, Śvetaketu." Chandogya Upanishad, 6.8 - 6.16
Chandogya Upanishad - Wikipedia