Although God revealed Himself as Trinity more clearly in the New Testament, He pointed forward to this dogma even in the Old Testament. For example, we see this in the mysterious meeting of Abraham and the three strangers. We will quote here from an older translation of the Bible to show a distinction which is lost in more modern translations. As we read these few lines, let us look for the oddity
And the Lord appeared unto him in the plains of Mamre: and he sat in the tent door in the heat of the day; And he lift up his eyes and looked, and, lo, three men stood by him: and when he saw them, he ran to meet them from the tent door, and bowed himself toward the ground, 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… (Genesis 18:1-4)
Here is the oddity. There are three men, but Abraham addresses them with a singular pronoun, i.e., Thou. In modern English the distinction between the informal Thou and the formal You is lost. Now-a-days we use you for all people. English formerly made this distinction which is preserved in other languages as we see in the German “du-sie” or the French “tu-vous” When the Fathers commented on the passage they see the use of the singular Thou to address three people as a hint of the Tripersonal unity of the Trinity. Taken by itself this interpretation may seem somewhat obscure but in context of the whole Old Testament it makes an important point.
It is interesting that this Old Testament Trinity is the basis of many icons. Sometimes when people talk of the Old Testament Trinity icon they think of one that shows God the Father as an old man, Jesus Christ as a young man and the Holy Spirit as a dove. This icon is very popular in Western Christian churches and is also found in many Orthodox Churches. As an object of piety and devotion, it should be respected. However, many scholars will state that a better icon of this scene is seen in the icon by St. Andre Rublev, commonly called the Hospitality of Abraham. An important council of the Russian Church held in 1551 said that only icons of the Rublev type should be used. The idea then is that God the Father never became a human being so trying to paint Him, one is bound to be subjective. By depicting the three strangers as angels the Rublev Trinity leads us to a more profound understanding of this passage.
Incidentally, there is some controversy about who is who in the icon. Some say that the center figure is the Father and the figure on the left represents the Son. Other interpretations hold that the center image is that of Christ with the Father on the left and the Holy Spirit on the right. But this is taking the icon in too literal a sense. An artist ultimately cannot paint a literal icon of the Trinity, so we have to respect the mystery.
However, if we want to know the Holy Trinity, we do so not by manipulating words or concepts, but by the experience of the Trinity through prayer, liturgy, and meditation or by reading the Bible. In the end, God is beyond all words and images but the works are concepts given to us by the church as stepping stones to know the one, true God.