One of the things about Christians that puzzled the Romans is that Christians were not an ethnos, a people. Jews were a people; Persians were a people; but Christians could be from any ethnic background.
In the first century, one worshiped the gods of one's family. Christianity involved a deliberate choice to worship the God revealed in Jesus of Nazareth, rather than the gods of one's family. In the case of Jews who became followers of Jesus, it involved a decision to recognize that the God revealed in the Torah was now fully and completely revealed in Jesus. In either case, one's ties to one's family were at least attenuated or (more often) completely severed.
These things hover in the background to Jesus' conversation with Nicodemus. "No one can see the Kingdom of God without being born from above." Jesus' words astonished Nicodemus because it was precisely through his birth, his connection to his family, that he hoped to see God's kingdom, and he had a firm Biblical basis for believing this. That was exactly what God had promised to Abram long, long ago. God's promise to Abram was not that the earth would be blessed through him or even that all human beings would be blessed but that the families of the earth would be blessed through him.
Paul was confused about this, too. In writing to the Christians at Rome, he argued that what God meant was not that the blessing would fall upon the physical descendants of Abraham but to those who shared his faith, in other words, his spiritual children. But that is not really what Genesis says.
Jesus' words to Nicodemus are still startling. "You must be born from above (or in the more familiar version, "born again")". They tell us that like Abram we, too, have to strike out into the unknown and if necessary, to leave family behind. But they may be especially good news in an age of alienation and estrangement. Jesus invites us into a new family. He tells us that there is a place for us, a community where we will be at home.
The Apostles' Creed is the basic baptismal statement. In the early church, the bishop would ask the shivering catechumen standing in the cold, running water, "Do you believe in God?" And she would reply, "I believe in God the Father" as the bishop poured water over her head. Three times the bishop would ask and three times the catechumen replied, "I believe." However, when the church decided to include the Nicene Creed in the liturgy, the faithful stated their faith not as "I believe" but as "WE believe". We enter the waters of baptism as individuals but we emerge from them as a new people, God's own people. We go into the font an I; we emerge a WE.