Baptism in the middle ages, according to Leithart, meant more than religious rite. It served the dual function of entry into the church and "civil registry". He brings this up to demonstrate the important connection between religion and society that existed until the 1600's. Baptism had a clear sociological import.
Why is this significant? Leithart contends that modern political theory, with its Lockean view of human nature as "hard atoms", makes a strong view of baptism difficult to maintain. In this individualistic political environment, we are "I" before we are "We" and even then we are "I" first - the individual is in the major key. Here, the deep and abiding sociological implications of baptism become more difficult to imagine.
Leithart counters this with an interesting sociological observation from sociologist Rosenstock-Huessy. We don't start as an "I". It's just not our experience. We all begin as "You". We don't develop as isolated individuals. Rather, we come to know who we are by what others (mom/dad/grandma) say to us. He says, "Our children only speak in the first person after they have been addressed in the second person . . . infants develop a sense of personal identity because we talk to them using names they didn't choose."
The importance of this point will be clearer in a bit. Suffice for now to say, if who we are defined by this relational interaction, then the implications of baptism come to the fore. In the rite of baptism our heavenly Father speaks to us and what he says forms who we are.