The first book of the Bible, Genesis, when God is creating the world He says:
“Let us make man in our image, after our likeness; and let them have dominion over the fish of the sea, and over the birds of the air, and over the cattle, and over all the earth, and over every creeping thing that creeps upon the earth.” (Gen 1:26)
In other words, God is saying “us” “our” and not simply “I” or “my”. It seems that there is a hit of plurality here. Jewish scholars and even some Christians would say that “our” and “us” is a plural of majesty. For example, in the ages of kings and emperors in Europe, they would send out orders and command in “our” name. The king might say “we are pleased” or “we are happy” or “we give our approval”. This may seem strange to us, but this custom still exists in some places even today. But again, we as Orthodox Christians, interpret these verses as a hit about the Trinity. Or in the book of the Prophet Isaiah, when Isaiah was in the temple he had this experience:
“In the year that King Uzzi’ah died I saw the Lord sitting upon a throne, high and lifted up; and his train filled the temple. Above him stood the seraphim; each had six wings: with two he covered his face, and with two he covered his feet, and with two he flew. And one called to another and said: “Holy, holy, holy is the Lord of hosts; the whole earth is full of his glory.” And the foundations of the thresholds shook at the voice of him who called, and the house was filled with smoke. (Is 6:1-4)
Orthodox theologians have always seen the triple “Holy, Holy, Holy” as pointing to the Trinity. Or in the Book of Psalms the church has always seen the “word and breath” as referring to Jesus Christ and the Holy Spirit.
“By the word of the Lord the heavens were made, and all their host by the breath of his mouth.” (Ps 33;6)
Of all these hints of the Trinity in the Old Testament, the one that has left the most traces in Orthodoxy is the s tory of the appearance of the three angels to Abraham at Mamre. Abraham is sitting outside his tent at Mamre,
“And the Lord appeared to him by the oaks of Mamre, as he sat at the door of his tent in the heat of the day. He lifted up his eyes and looked, and behold, three men stood in front of him. When he saw them, he ran from the tent door to meet them, and bowed himself to the earth, and said, “My lord, if I have found favor in your sight, do not pass by your servant. Let a little water be brought, and wash your feet, and rest yourselves under the tree, while I fetch a morsel of bread, that you may refresh yourselves, and after that you may pass on — since you have come to your servant.” So they said, “Do as you have said.” (Gen 18:1-5)
At first the casual reader might miss the Trinitarian reference. There are three men (angels) who approach Abraham. However, Abraham addresses them as “my lord” in the singular. He does not say “My lords” in the plural. This is even clearer in an older translation,
“And said, My Lord, if now I have found favor in thy sight, pass not away, I pray thee, from thy servant: Let a little water, I pray you, be fetched, and wash your feet, and rest yourselves under the tree.”
We see here that Abraham addresses the three angels as “thy”. We no longer use this word in modern English, but it is in the singular. This distinction exists in many languages between the singular you (thee) and the plural. So, we see here clearly that Abraham is addressing the three angels as “Thy” in the singular. However, in the next verse Abraham addresses them as You in the plural.
Orthodox theology has always seen this alternation between singular and plural as a prefiguration of the Holy Trinity. Obviously when Jews read the passage they do not see a reference to the Trinity. But when when the Christians read it, we read it in light of the New Testament. In other words, for us Christians, it is the theme in both Testaments.
This passage has also influenced iconography. We cannot draw an image of God the Father, because the Father is invisible and of course, the Holy Spirit cannot be depicted as a human being. Therefore, this icon often depicted in this passage has become the standard icon of the Trinity. It has to be admitted that one sometimes sees icons which depict the Father as an old man. This kind of icon is not “heretical’ but it is a later development. Probably the greatest icon of the Trinity is Andre Rublev’s, made in the 15th century. It is now in the Tretyakov Gallery in Moscow and has influenced the making of Trinity icons ever since.