This Monday, I dialogued with Muslims, Jews, and Unitarians in an online comments section. How’d it go? A Muslim man accused me of an “unforgivable sin” for espousing Trinitarianism. (I thought: “If that’s literally true, then that makes me less inclined to become Muslim. I mean, why bother?”) But the commenters were generally thoughtful and kind.
The blogger who hosts the website had written, “The Jews had no idea of the Trinity. Their faith was centred in the Shema: a unitary monotheistic confession. Jesus clearly affirmed that very same unitary monotheism in Mark 12:29. [“Jesus replied, ‘The first (commandment in the law) is this: “Hear, O Israel! The Lord our God is Lord is one!”] How is it that Christians today have abandoned their rabbi on this point?” I felt moved to reply and what follows is based upon my responses.
Faithful Jews recite the Shema prayer each morning and evening, quoting Deuteronomy 6:4: “Hear, O Israel: The LORD our God, the LORD is one.” Is the oneness professed in this passage of God’s word irreconcilable with Trinitarian belief?
In declaring that “the Lord is one,” the Hebrew passage employs the word “echad” for “one.” Echad is often used to mean singularity, but sometimes the same word denotes a unified entity. For instance, in the Garden, “For this reason a man will leave his father and mother and be united to his wife, and they will become one (echad) flesh.” And again, at the Tower of Babel, “If now, while they are one (echad) people and all have the same language, they have started to do this, nothing they presume to do will be out of their reach.”
God could have selected a different word to be inspired for this passage, but the one He chose allows a providential flexibility. Echad permits the unified oneness of the Persons of the Trinity without requiring this reading from the Jewish generations who came before Christ. So, contrary to the blogger’s claim, when Jesus quotes the Shema it is not clear that He is affirming the very same unitary monotheism assumed by his ancestors. Interestingly, the “oneness” of God taught in Deuteronomy 6:4 leads to the conclusion that we ought to love God with the unified oneness of three aspects of ourselves. The immediately following verse reads: “Therefore, you shall love the LORD, your God, with your whole heart, and with your whole being, and with your whole strength.”
A Unitarian (a Christian who asserts God’s unity and rejects the doctrine of the Trinity) found this last observation interesting and asked me to recommend a book that makes a great case for the Trinity. To him, I replied:
I would suggest approaching the Holy Trinity in the same way the first Christians came to this knowledge; through Jesus of Nazareth. Some dismiss the Christology of John’s Gospel as later theological development, but even the Gospels thought to be written earlier show Jesus doing and saying things only God could rightly do (e.g., forgiving sins, declaring himself lord of the Sabbath, demanding an absolute total commitment to himself, etc.) A book I recommend that explores this is Pope Benedict XVI’s 2007 “Jesus of Nazareth (Part 1)” In it, Benedict spends a good deal of time discussing Rabbi Jacob Neusner’s book, “A Rabbi Talks with Jesus.” The central issue that prevents that rabbi from believing in Jesus is the same scandal that led Jesus to his death: his revealing himself as God.

To objections at Christians detecting in the Shema something which no Jews had previously held–indications of the Trinity–I answered:
In the course of the Jewish Scriptures, we can see God developing humanity along; from polytheism to monotheism, from polygamy to monogamy, from blood vengeance to “an eye for an eye.” That the LORD was not just one god among the many gods–but the only God, was a revelation His people learned over time. (For example, Moses must ask of God in Exodus 3:13: “If I go to the Israelites and say to them, ‘The God of your ancestors has sent me to you,’ and they ask me, ‘What is his name?’ what do I tell them?”) When Jesus comes He extends the revelation further; “Love your enemies,” “What God has joined let no man separate,” “The Father and I are one.” My point is this: To argue Christian beliefs cannot be true because they were not previously known among Jews is like saying there cannot be only one God because this was not clear to the Patriarchs; Abraham, Isaac, and Jacob/Israel.
One Muslim asked whether the Old Testament prophets who did not know about the Trinity would therefore be worshiping an incomplete God. I answered:
All analogies touching on the Trinity fall short, but imagine being introduced to a friendly and engaging man at a dinner-party. In the course of your conversation you learn that he is a doctor, married, and has three kids. Now these things were true about the man from the first moment you knew him but you came to know him more fully with time. Likewise, God has always been a Trinity of Persons; Father, Son, and Holy Spirit; acting, speaking, and revealing throughout history. Abraham and the prophets’ understanding of God were not as filled-out as in later generations, but they did indeed know and love and worship God. Of course, the parallel I’m trying to draw is not that God is one person wearing three different hats like that doctor-husband-father (which is the heresy of modalism.) I’m noting how Abraham and the prophets could enjoy true relationship with the Holy Trinity without yet knowing of that doctrine.

A modern icon based upon Andrei Rublev’s “The Trinity” (also called “The Hospitality of Abraham“) from the fifteenth century.
Consider the interesting episode of Abraham’s three visitors in Genesis 18: “The LORD appeared to Abraham by the oak of Mamre… Looking up, he saw three men standing near him.” Now two of this trio are later called angels (Genesis 19:1) but more precisely these are “messengers,” and the Son and the Holy Spirit do indeed serve the Father this way, revealing God the Father and his will among men. Did Abraham perceive in his guests what Christians suspect in retrospect, that this was a manifestation of the Holy Trinity? Maybe not, yet Abraham could still commune in God’s presence.
I think something that trips people up about Christianity is imagining God the Father as the sole Divine Person in the Old Testament, with the Son and the Holy Spirit only appearing later in the New. However, if the Trinity is true it has always been true, and the three Persons (possessing the same Divine Essence the prophets praised) have been active in the affairs of mankind throughout history. Christians reflect back on the Jewish Scriptures and see the Persons of the Trinity at work together. Our Nicene Creed professes that the Holy Spirit has “spoken through the prophets.” Look at the episodes where a “messenger” speaks in the divine first-person (e.g., Genesis 22:12, Judges 6:16 & 13:21-22) I would say Abraham and the prophets’ experiences of God were Trinitarian even if they did not fully grasp it then. I believe the same is true for all today.