Biblical Unitarian: Rambam

Rambam, or Maimonides, was a Jewish philosopher and educator, and fierce defender of Jewish doctrines. Rambam did not write extensively on the Trinity since this was not a subject in Judaism, but he did have a few things to say about it in his letter on the resurrection:

“It is not rare that a person aims to expound the intent of some conclusions clearly and explicitly, makes an effort to reject doubts and eliminate far-fetched interpretations, and yet the unbalanced will draw the reverse judgment of the conclusion he sought to clarify. Some such thing occured even to one of God’s declarations. When the chief of the prophets wished by order of God to teach us that He is One, without associates, and to remove from our hearts those wrong doctrines that the Dualists propound, he proclaimed this fundamental: The Lord is our God, the Lord alone [Deut. 6:4]. But the Christians utilized this verse to prove that God is one of three, teaching that Lord, our God, the Lord makes three names, all followed by one, which indicates that they are three and that the three are one. Far be God from what they say in their ignorance. If this is what happened to God’s proclamation, it is much more likely and to be expected to happen to statements by humans.”

Rambam resented the fact Christians were “finding” the Trinity in places that made no grammatical sense. For this reason, Rambam replaced the word “echad,” meaning “one,” with “yachid,” meaning “alone”; thus, his translation “The Lord is our God, the Lord alone” (most translate as “the Lord is One“). His hope was to clearly convey the true meaning of the text, and not impose some esoteric message that is simply not in the text.

The source used can be purchased here:
Epistles of Maimonides: Crisis and Leadership

Biblical Unitarian: Rashi

There have been many attempts to find the Trinity in the Old Testament. One of the most cited is the “Us” language used in Genesis. However, this was disproven hundreds of years ago by the rabbis.

The following is Rashi‘s commentary on Genesis 1:26:

“Let us make man: From here we learn the humility of the Holy One, blessed be He. Since man was created in the likeness of the angels, and they would envy him, He consulted them. And when He judges kings, He consults with His Heavenly household, for so we find regarding Ahab, that Micah said to him, (I Kings 22:19): “I saw the Lord seated on His throne, and all the host of heaven were standing by Him, on His right and on His left.” Now do “left” or “right” apply to Him?! But rather, [the passage means that] these [angels] were standing on the right to defend, and these [angels] were standing on the left to prosecute. Likewise, (Dan. 4:14): “By the decree of the destructive angels is the matter, and by the word of the holy ones is the edict.” Here too, He took counsel with His heavenly household. He said to them, “Among the heavenly beings, there are some in My likeness. If there are none in My likeness among the earthly beings, there will be envy among the creatures of the Creation. ” – [from Tanchuma, Shemoth 18; Gen. Rabbah 8:11, 14:13]. Let us make man: Even though they [the angels] did not assist Him in His creation, and there is an opportunity for the heretics to rebel (to misconstrue the plural as a basis for their heresies), Scripture did not hesitate to teach proper conduct and the trait of humility, that a great person should consult with and receive permission from a smaller one. Had it been written: “I shall make man,” we would not have learned that He was speaking with His tribunal, but to Himself. And the refutation to the heretics is written alongside it [i. e., in the following verse:]“And God created (וַיִּבְרָא) ,” and it does not say, “and they created וַיִּבְרְאוּ.” – [from Gen. Rabbah 8:9].”

These sources can be purchased here:
Sapirstein Edition Rashi: The Torah with Rashi’s Commentary Translated, Annotated and Elucidated, Vol. 1 [Student Size], Genesis [Bereishis]

Midrash Rabbah (10 Vol. Set)