Christianity changed rapidly and radically in the post-apostolic era.
A basic knowledge of early Christian history will greatly improve your understanding of these doctrinal and ecclesiastical developments, and the influences that drove them.
Chadwick, The Early Church, 1993.
Davidson, The Birth of the Church, 2005.
Davis, The First Seven Ecumenical Councils, 327-787, 1992.
Frend, The Early Church: From the beginnings to 461, 1982.
Frend & Stevenson, Creeds, Councils and Controversies, 2011.
Frend & Stevenson, A New Eusebius, 2013.
Hall, Doctrine and Practice in the Early Church, 2011.