"Hear, O Israel, Adonai Eloheinu Adonai is one. These three are one. How can the three Names be one? Only through the perception of faith; in the vision of the Holy Spirit, in the beholding of the hidden eye alone.…So it is with the mystery of the threefold Divine manifestations designated by Adonai Eloheinu Adonai—three which yet form one unity."
A Christian quote? No. This is taken from the Zohar, an ancient book of Jewish mysticism. While the Zohar is somewhat esoteric and most contemporary Jews don't study it, there are other ancient Jewish writings that refer to God's plurality as well.
The Ancient sages struggled with several portions of the Hebrew Scriptures and their implications of the "compound unity" of God. This study shows how ancient (and often, modern) Jewish thought confirms the fact that there is one God whose oneness is complex.