The anti-missionary indicates that the concept of the Trinity is a pagan concept. However, the rabbis express two views of the Trinity. The anti-missionary has presented one point of view. However, not all in the Jewish community share this view. For example, Rabbi Alfred J. Kolatch, from his book The Second Jewish Book of Why, asks the question, “Why does Jewish law not consider the Christian belief in the Trinity to be idolatrous?” He then states that there are two ways to view the Trinity in the Jewish community. One view is the view of Maimonides, which the anti-missionary subscribes to, that the Trinity is an idolatrous concept. However, there is also a second view:
Scholars such as the French-born Rabbenu Tam (1100-1171), the grandson of Rashi who spent his life in Christian Europe, accepted the view of Christian theologians, who explained that the Trinity is consistent with the concept of one God. To these theologians the three personages are part of the one God; they are not individual gods. Just as spokes of a wheel are not in themselves wheels but components that are integral to the actual wheel, so the three personages are not gods but together they comprise the one God.
Under a different question, we find this related comment by Rabbi Kolatch:
This view of Maimonides (that the Trinity is idolatrous) does not appear in our editions of the Mishneh Torah and was never accepted by normative Judaism. Beginning with Rabbenu Tam, a contemporary of Maimonides, almost all authorities agree that despite their belief in a Trinity Christians are monotheists. Rabbenu Tam notes in his commentary that despite outer appearances Christians truly believe in one God, as do Jews.
In the footnote to the comment above, we find these words:
Bechorot 2b. This opinion has prevailed over the centuries. The great German rabbinic scholar Jacob Israel Emden (1697-1776) expressed the attitude of most scholars when he wrote that Christians cannot be considered idol worshippers and that it’s incumbent upon every Jew to befriend Christians in their hour of need, as was taught by the Sages of the Talmud (Gittin 61a).
This position is echoed by the National Jewish Scholars Project. They issued a document entitled A Jewish Statement on Christians and Christianity. In that document, we find this assessment:
Jews and Christians worship the same God. Before the rise of Christianity, Jews were the only worshippers of the God of Israel. But Christians also worship the God of Abraham, Isaac, and Jacob; creator of heaven and earth. While Christian worship is not a viable religious choice for Jews, (please realize that HaDavar does not accept that statement) as Jewish theologians we rejoice that, through Christianity, hundreds of millions of people have entered into relationship with the God of Israel.
The preceding statement is rife with implications, one of which deals with the nature of God. If we are dealing with the same God, then what is He like? Another implication of that statement affects the idea of idolatry and polytheism. If we are dealing with the same God, then we are not dealing with paganism. The Trinity is not a pagan concept.
In contrast, there is solid evidence in the Jewish community to the contrary. The concept of the Trinity is not rejected outright, and we must look at what the Bible has to say in as neutral a manner as possible. We believe that the concept of the Trinity is defensible from the Biblical data.
Now for one final comment before we actually get into the biblical text about the relationship between paganism and the Trinity. The Trinity is biblical truth that has become distorted by people who do not have access to or interest in God’s revelation. The Bible is the only source of accurate revelation about God found in this world. However, as mankind scattered over the face of the earth (Gen. 11:8), the knowledge of God became distorted and forgotten. That revelation was eventually entrusted to the care and keeping of the Jewish people (Gen. 12, Rom. 3:2) where it has been safeguarded ever since.
The Brit Chadashah (New Testament) puts it this way:
And in the generations gone by He permitted all the nations to go their own ways; and yet He did not leave Himself without witness … 
In Acts 4, Rabbi Shaul, the Apostle Paul, is speaking to a Gentile crowd in the city of Lystra in Asia Minor. His comment indicates that as the nations went their own way, they strayed from the knowledge of the truth. Truth degenerated into partial truth, partial truth disintegrated into untruth. The correct understanding of the one true and living God degenerated into polytheism and idolatry. Polytheism is simply a horribly distorted and barely recognizable perversion of biblical truth.
All men once knew that the God of Abraham, Isaac, and Jacob was a complex or compound unity. The Bible, which we, the Jewish people, have faithfully safeguarded for millennia, contains the true and accurate revelation about God, His program for humanity, and the Messiah. We should examine that data objectively and thoroughly.
Another biblical principle to keep in mind is that God’s revelation is “progressive.” We learn more and more about God and His plan for the Jewish people and humanity as God unfolds history and His revelation to us. For example: no one who lived previous to Isaiah would know that the Messiah would die as a guilt offering. No one knew until that fact was revealed to Isaiah in Isaiah 53:10. My point in saying this is to acknowledge that the complex, indivisible unity of God is not clearly revealed in Tanakh (Old Testament). However, the complex, indivisible unity of God is fully revealed in the New Testament. The revelation in the Brit Chadashah is not contradictory to the revelation found in Tanakh. If it were contradictory, we would not accept it. This is why we do not accept other claims such as Mormonism, Islam, Buddhism, Hinduism, etc. Their writings contain material contradictory to Tanakh. The concept of the Trinity is defensible from Tanakh due to a number of lines of evidence. Here are some of them.