An interesting, and beautiful, story unfolds in Genesis 15, and if you read through it took quickly, you're likely to miss it altogether.
It is a story of Abram - before he became Abraham, before he had a son, before he entered the land that God was giving him. It's a story that, not accidentally, comes right after Abram is blessed by Melchizedek, who is a priest even though there is not yet a Temple. And it is a story in which all three persons of the Trinity - Father, Son, and Holy Ghost - make an appearance, thousands of years and hundreds of pages before Jesus is even born or the Spirit falls on Pentecost.
So, you know, it's a pretty cool story.
Abram is having a vision about all the things that God has promised him, things, no doubt, that he and Melchizedek just discussed at some length in his blessing. And all of a sudden, "the word of the Lord came" to Abram, not once, but twice (at least). Most of us read this and think that Abram heard here a message from God, perhaps from some booming voice or some kind of thunder or something. But Abram's response to this "word" gives Him away - "And Abram said, Lord God...."
Remember what John said in the very beginning of his gospel? "In the beginning was the word, and the word was with God, and the word was God." Jesus is the word of God; He always has been. Although the Hebrew and the Greek terms are different, the word used here means roughly the same thing as logos, which is the more familiar word from the Greek.
So here is Abram and the "word of the Lord" comes to him and he responds by saying, "Lord God." This is no booming voice or thunder or conversation with nothing but thin air; this is Jesus Himself. How could it be any other?
The scene continues to develop a little bit, promises given, covenants confirmed, and Abram prepares a sacrifice for the Lord who has made such an extravagant promise to him. He lays the pieces of his sacrifice on an open stone, then starts to nod off next to them. While sleeping, the Scriptures tell us he was in a deep sleep, "a smoking furnace and a burning lamp" passed between the pieces of his offering; other versions say that a fire arose that consumed them.
And where do we see such holy fire in the Scriptures? Why, in the Holy Spirit, of course! In Acts when the Spirit comes upon the people, it comes in tongues of fire. It's God's Spirit that consumes Abram's offerings or, as we call Him, the Holy Spirit. Right here, right in the early pages of Genesis.
Immediately following the burning flame of the Spirit that consumes Abram's offerings, God the Father makes His entrance into this Trinitarian scene. For it says, "In the same day the Lord made a covenant with Abram...." And it is only God the Father who makes covenants, for it is God the Father whose mighty hand created all things, for it is God the Father who determines the direction of all things, for it is God the Father who sets the course of all things. It is God the Father who decides what to do with the earth, and in this case, He covenants a choice portion of it to Abram.
Thus, here we are - all the back in Genesis 15 with echoes of the entire story of God all wrapped up in His covenant with one man. All the way back at Abram, thousands of years and hundreds of pages before we could even fathom it, Father, Son, and Holy Ghost all enter in to the narrative and reveal themselves - the Word, the Fire, the Father.