Part I: The Trinity in the Tanakh and in Jewish Sources

God is Spirit

As we have just said, God is transcendent - entirely distinct from creation. He is not physical or corporeal like man. He is spirit and has no body. He is unchanging, infinite, eternal (CCC 42, 212, 300). At the same time, God immanent: He actively upholds and sustains creation at every moment (CCC 300-301).  In the words of the psalmist:

Where can I go from Your Spirit? Or where can I flee from Your presence? If I ascend into heaven, You are there; If I make my bed in hell, behold, You are there. (Ps 139:7-8)

One God

The oneness of God is emphatically affirmed in the Old Testament, with the Sh'ma forming the timeless backbone of the Jewish creed:

"Hear, O Israel: The Lord our God, the Lord is one!" (Deut 6:4)

"I am the Lord, and there is no other; there is no God besides Me." (Isa 45:5, also 46:9)

Yet at the same time the oneness of God is not described as an absolutely singular entity. The Hebrew word for one, "echad," is often used in the Hebrew Scriptures to describe a composite unity made up of two or more parts:

"And it was evening, and it was morning – one day (yom echad). (Gen 1:5)

"Therefore a man shall leave his father and mother and be joined to his wife, and they shall become one flesh (basar echad)." (Gen 2:24)

The whole assembly was as one, forty-two thousand three hundred and sixty.  (Ez 2:64)

The Plurality of God in the OT

In addition, there are several passages in the Tanakh which portray God as a composite unity. The most common example is found in almost every page of the Bible: the very name for God in Hebrew, "Elohim," is a plural noun which can also be translated as "gods." Other examples are more explicit:

"Let us make man in our image, according to our likeness". (Gen 1:26)

"Behold, the man has become like one of us". (Gen 3:22)

"And the Lord said: …Come, let us go down and confuse their language…"  (Gen 11:6-7)

"God caused me to wander from my father’s house…"  (Gen 20:13)  
In the original Hebrew, the verb "caused me to wander" (התעו, hit’u) is in the plural. This, however, is lost in the translation. 

"There is a God who judges (plural) the earth." [Literally: "There are Gods who judge the earth."] (Ps 58:12)
Here also the subtlety is lost in the translation: in Hebrew, the verb "judges" is in the plural (יש אלוהים שפטים בארץ).

"The LORD said to my Lord, ‘Sit at My right hand, till I make Your enemies Your footstool.’" (Ps 110:1)

"Remember your creators in the days of your youth."  (Eccl 12:1)
In the Hebrew, the word "your creators" is in the plural (בוראיך).

One God, Three Persons

Other passages seem to portray a number of persons, two or three at the most, within God himself. Notice in the following passages that God is speaking, yet He seems to be speaking about "another" God - and His Spirit:

"Thus says the Lord, the King of Israel (1), and his Redeemer the Lord of hosts (2)". (Isa 44:6)
 
"Listen to Me, O Jacob, and Israel, My called: I am He, I am the First, I am also the Last… From the time that it was, I (2) was there. And now the Lord God (1) and His Spirit (3) have sent Me (2)".  (Isa 48:12, 16)

"I will dwell in your midst, says the Lord (2)…then you will know that the Lord of hosts (1) has sent Me (2) to you." (Zech 2:10-11)

The Mystery of Three in the Zohar

The medieval Zohar, the most important book of Jewish mysticism, describes the tri-unity of the Godhead in quite explicit terms: [emphases added]

"Come and see the mystery in the word YHWH: there are three degrees, and each degree exists on its own; nevertheless, they are one, and they are united in a way that they cannot be separated one from another."  (Zohar, vol. 3, p. 65, Amsterdam edition)

"The same one, Holy and Ancient of Days is revealed in three heads, contained in one head, and it is the head that is exalted three times."  (Zohar, vol. 3, p. 288)

"God is the artist who dwells above… God is the artist below, and this is the Shekhinah on the earth… The God of creation commanded, and immediately the artist worked and did according to his Word.  When God was revealed in the world of the separate ones (the angels), the artist said to the Lord of the buildings: ‘Let us make man in our image, according to our likeness.’" (Zohar, Genesis, p. 22)

"The three spirits are united in the One.  The spirit located below is called "the Holy Spirit."  The middle spirit is the central pillar, called "spirit of wisdom and understanding"… The uppermost spirit is hidden in secrecy, and in it exist all holy spirits and all sources of light."  (Zohar, Genesis, p. 15)

"YHWH, Adonai, and the Shekhinah, they are the Holy One, blessed be He, and the Shekhinah is located between two extremities: YHWH on its right side, and Adonai on its left side.  And they are an illuminating vision.  But without the Shekhinah the vision will be obscure.  In one righteous, the Shekhinah, they are One."  (Tikunei haZohar, p. 66)

The "Middle Spirit": the Word of God

These passages from the Zohar portray God as a tri-unity called by different names: YHWH, Adonai, and the Shekhinah; or the uppermost spirit, the middle spirit, and the spirit below (the Holy Spirit). The "middle spirit" could well be associated with the eternal Word of God, present before creation, described in the Aramaic Targumim. The Targumim are Jewish translations of the Bible into Aramaic that date back to the early centuries of the Christian era. We will now see that several passages which directly refer to God in the Bible refer to the "Word of the Lord" in the Targum. The following verses will be presented in pairs. The first passage is the verse from Scripture, and the second is its Aramaic translation from the Targum:

"So God created man in His own image; in the image of God He created him; male and female He created them." (Gen 1:27)
"And the Word of the Lord created man in His likeness, in the likeness of the presence of the Lord He created him, the male and his yoke-fellow He created them." (Jerusalem Targum)

"And God spoke all these words, saying…" (Ex 20:1)
"And the Word of the Lord spoke all the excellency of these words saying..." (Jerusalem Targum)

"Rise up, O LORD! Let Your enemies be scattered." (Num 10:35)
"Arise now, O Word of the Lord, in the power of Thy might, and let the adversaries of Thy people be scattered…" (Jerusalem Targum)

"Israel shall be saved by the LORD with an everlasting salvation… In the LORD all the descendants of Israel shall be justified, and shall glory."  (Isa 45:17,25)
"Israel shall be saved by the Word of the Lord with an everlasting salvation… in the Word of the Lord all the descendants of Israel shall be justified and shall glory." (Targum Jonathan)

As we have seen in the article on the divinity of the Messiah, God also appears as the "angel of the Lord" in the Old Testament. The angel of the Lord is also called "Metatron" and "Son of God" in the Zohar and other Jewish mystical works. This "Son of God" is plausibly identical with the "Word of the Lord" spoken of in the Targum.

God's Spirit

In addition to the "middle spirit," who is the eternal Word of God, we also encounter God's Spirit - the Holy Spirit - in the Tanakh, who also seems to be somehow distinct from God Himself:

"And the Spirit of God was hovering over the face of the waters." (Gen 1:2)

"Do not cast me away from Your presence, and do not take Your Holy Spirit from me." (Ps 51:11)

"There shall come forth a Rod from the stem of Jesse, and a Branch shall grow out of his roots. The Spirit of the LORD shall rest upon Him, the Spirit of wisdom and understanding, the Spirit of counsel and might, the Spirit of knowledge and of the fear of the LORD." (Isa 11:1-2)

The Spirit of the Lord GOD is upon Me… (Isa 61:1)

In summary, we see that even though God's self-disclosure in the Tanakh primarily reveals and emphasizes His unity, this unity is by no means absolutely singular, but rather a composite unity - and this composite unity is confirmed by later Jewish sources such as the targumim and the Zohar.