Ignatius of Antioch

Ignatius wrote seven letters on his way to be eaten by lions in Rome. Read them and the second-century church stops being abstract. He warns against early heretics, defends the bodily resurrection, and is the first writer to describe a single bishop leading a city's church — the model that became universal. He also coined 'Catholic' (katholike, universal) as a description of the church. Everything you'll later argue about with Catholics, Orthodox, and Protestants traces some root through him.