Maimonides 13 Principles of Faith: 2. Unity of God

2. I believe with perfect faith that the Creator, Blessed be His Name, is One, and that there is no unity in any manner like His, and that He alone is our God, who was, and is, and will be.

In Orthodox Judaism
No Jewish teacher has openly disputed that God is one and unified. The system of kabbalah, however, does speak of God as having a complex unity. At the heart is the Ein Sof, the divine essence which is unknown to us. Through the 10 sefirot, the Ein Sof is manifested to us.

R. Isaac ben Sheshet said that Christians believe in three, but kabbalists believe in ten, in the same sense. R. Abraham Abulafia said that the system of sefirot in kabbalah said that they multiply God into ten in exactly the same way that Trinitarianism multiplies God into three. Therefore if kabbalah is acceptable in Orthodox Jewish theology, then so must Trinitarian monotheism be as well.

Many of the kabbalists argued that the sefirot are part of God's essence. R. Moses Cordovero said that God "emanated ten Sefirot, which are from his essence, are one with him and are all one complete unity."

Kabbalists have never regarded the doctrine of the sefirot as a violation of divine unity, just as Christian theologians have never regarded the persons as a violation of divine unity.

R. Isaac Lopes of Aleppo believed that the God of Israel is not the First Cause, but the first created being, similar to the Demiurge of ancient Greek thought.

The anonymously written kabbalistic work Ma'arekhet ha'elohut, written in the vicinity of the 14th century, identifies the God of Scripture not with the Ein Sof, but with the first or second of the sefirot. Even if there are not two Gods in this view, there are two different supreme powers, which can at least be described as to persons.

In line with this, R. Isaac ibn Latif described the God of the Bible as the First Created Being and the creator of all else, very similar to the Arian view of Jesus.

R. Jacob the Nazirite directed the first three and last three benedictions of his amidah prayer to the sefirah called binah, and the others toward tiferet. In other words, he directed his prayers to specific sefirot, just as many Christians pray to the Father or to the Son or to the Holy Spirit.

R. Abraham ben David of Posquieres directed the first and last three benedictions to the Ein Sof, and the rest to the Creator, which he distinguises from the Ein Sof.

According to R. Joseph ben Shalom of Barcelona, the sefirot themselves pray to the Ein Sof. According to R. Azriel of Gerona, all sefirot except for the first one had a beginning in time, while the Ein Sof and the keter sefirah are eternal.

R. Isaac Pilitz argued that while the Ein Sof knows the future, the sefirot, which run the world, do not.

Some kabbalists, such as R. Abraham Epstein, also pray to the "unique cherub" which is an anthropomorphic entity which emanates from God.

In Christian Theology
In Christian theology, the Triune God is one, and there is no unity like his unity. This is why analogies break down.

Trinitarianism states that there is one God, and from this one God are manifest three divine persons. There are two main schools regarding this idea. Social trinitarianism emphasizes the distinction between the persons, and Latin trinitarianism, which emphasizes divine unity.

William Lane Craig has an excellent video series on the different views of the Trinity. His own view is that God is one mind with multiple centers of self-consciousness. After all, there are times when you are unconscious, and have no center of self-consciousness, and therefore the "self" is not identical to the mind.

Thomas Aquinas took a different view. On his view, God's oneness (echad) and unity (yachid), and immutability, are so extreme that God not only lacks parts, but also lacks properties. This view is called divine simplicity, and is a form of divine unity far more extreme than most Orthodox Jewish theologians held. Yet, he still had a fully developed view of both the Trinity and the Incarnation, which were compatible with this extreme view of both unity and immutability.

Again, the rabbis have no basis for rejecting a view like that of Aquinas, or even Craig, as heretical. Orthodox Jewish theologians have held to much more extreme views of God's multiplicity than has been held in Christian theology.