God has spoken once, twice have I heard this

This means that God has said this more than once.

power belongs to God

God being characterized by power is spoken of as if power belonged to him. The abstract noun "power" can be translated with an adjective. Alternate translation: "God is powerful" (See: rc://en/ta/man/jit/figs-metaphor and rc://en/ta/man/jit/figs-abstractnouns)