Christian Belief
The foundation of Christian theology includes belief that G-d exists as a Trinity, and that Jesus is the bodily incarnation of G-d and acts as a mediator between G-d and man. Hebrew Christian missionaries claim that this theology is totally compatible with Judaism.
Jewish standpoint
Judaism maintains that although certain beliefs may be permissible for non-Jews, they are not acceptable for Jews. The Christian theology concerning G-d is one example of a belief that is absolutely forbidden to Jews according to the Hebrew Bible, as the following biblical sources demonstrates:
“Hear O Israel, The Lord our G-d, the Lord is One.” (Deuteronomy 6:4)
G-d’s absolute Oneness
The commandment to believe in G-d’s absolute Oneness was given specifically to the children of Israel (the Jewish people).
The concept expressed in this verse (Deuteronomy 6:4) not only refutes the plurality of G-ds, but also asserts that G-d is the only true existence.
Biblically
G-d is not only infinite, but He transcends time, space and matter. G-d has no beginning and no end, as it states:
“I am the first and I am the last and besides Me there is no other.” (Isaiah 44:6)
While Judaism believes that G-d manifests Himself to His creation (humanity) in many ways, (i.e. as a judge or a protector) G-d’s essence itself is indivisible and therefore without any possibility of distinction. Something that transcends both time and space cannot be described as consisting of three different aspects. The moment we attribute any such distinctions to G-d’s essence, we negate His absolute Oneness and unity.
One vs. Unity
Missionaries incorrectly argue that the use in Deuteronomy 6:4 of the Hebrew word for One (Echad), rather than the word unique (Yachid), teaches that G-d is a “composite unity” instead of an “absolute unity.” They claim that the Trinity is a composite unity, similar to a physical object that includes many different individual aspects, (eg. one pen, composed of ink, plastic and metal). This reasoning is incorrect, since physical objects that exist within the context of time and space cannot be used to describe G-d who transcends these dimensions. Prior to Creation, G-d was alone and concepts of time, space and the plurality of numbers did not exist. The term unique (Yachid), correctly describes G-d’s existence prior to Creation since it indicates the absence of any plurality or of rapport with any created object.
What does it mean that God is one, when His name is Elohim?
Christian View The word "one" used here is "echad." Christians point to other instances where "echad" is used to denote a compound unity, and as such, they declare that every time "echad" is used, that it denotes a compound unity, and therefore, The Lord must be a compound unity, proving that the Trinity is in the Torah!
Jewish Response The word "echad" in Hebrew actually works in the same way the word "one" does in English. It can mean either a single unity or a compound unity. These Christians are very quick to point to Genesis 1:5, fail to point out verses like these:
"And Pharaoh sent, and, behold, there was not one of the cattle of the Israelites dead. And the heart of Pharaoh was hardened, and he did not let the people go." Exodus 9:7
"And it came to pass, while they were in the way, that tidings came to David, saying, Absalom hath slain all the king's sons, and there is not one of them left." 2 Samuel 13:30
"So shall we come upon him in some place where he shall be found, and we will light upon him as the dew falleth on the ground: and of him and of all the men that [are] with him there shall not be left so much as one." 2 Samuel 17:12
"There is one [alone], and [there is] not a second; yea, he hath neither child nor brother: yet [is there] no end of all his labour; neither is his eye satisfied with riches; neither [saith he], For whom do I labour, and bereave my soul of good? This [is] also vanity, yea, it [is] a sore travail." Ecclesiastes 4:8