He will testify of Me. There is but one, and that is God. Some people feel that way about the doctrine of the Trinity, noting that the word isn't even in the Bible. This truth, however, is clearly taught in Scripture. The idea of a triune God is biblically sound and makes sense.
Love always needs an object.
If God were an eternal, absolute unity, without the distinction of persons within Himself, eternal love could not exist. But because God is a Trinity, the Father, Son, and Holy Spirit can express love to one another. There's perfect unity in the Trinity.