In the first reading God said, “You will be my people, and I will be your God.” This repeats what we find in Exodus, Leviticus and other places where God said, “You will be my people and I will be your God.” It could put you in mind of the Catholic marriage ceremony in which the priest asks, “Have you come here freely to give yourselves to each other in marriage.”
Those are all words by which parties bind themselves to each other in covenants. A covenant is a ceremony by which two parties unite as one.
The first Biblical covenant ritual was a primitive one. In Abraham’s time there were no governments, and with wandering shepherds competing for water and grass rights, they devised a ritual for enacting a covenant.
They would dig a four feet deep trench across a field, and they would kill and cut in two a heifer, a goat, a ram; putting the halves of the animals on opposite sides of the ditch. Then, with the followers of each chief gathered in the woods at opposite ends of the ditch, the chiefs would approach each other, each saying, let us be cut in two like these animals if we violate peace with each other.
Chapter Fifteen of Genesis describes how when God proposed making a covenant with him, Abraham prepared one of those primitive ceremonies for it. He dug the ditch, split the animals, then waited. With the dark, God, in the form of a torch, came at him through the trench, touching the animal halves.
In Chapter Twenty-Four of Exodus Moses at Sinaii staged a more sophisticated covenant ceremony for the people and God. He had young men carry in brass bowls of cattle blood, telling them to sprinkle part of it on the people, and part on God’s altar. With their belief that blood was life itself, this sharing of blood made them one with each other and with God. However, since God could not become one with an unclean people, ad the young men sprinkled the blood, Moses had the people promise aloud to keep each of the commandments.
Jeremiah later promised that God would enact a new covenant with the people. This happened at the Last Supper when Jesus used his own blood to unite us with each other and with him. He replaced the Ten Commandments of the Old Covenant with the new commandment. To become one with him we must love each other as he has loved us.