Is there but one God?

Yes; there is but one God.

Why can there be but one God?

There can be but one God, because God, being supreme and infinite, cannot have an equal.

How many persons are there in the Godhead?

There are three persons in the Godhead, the Father, the Son, and the Holy Spirit; and these three are one God, the same in essence, equal in power and glory. (1 Corinthians 8:6; John 10:30; John 14:9; Acts...