The world`s oldest autograph by a Christian is in Basel
(PHYSorg.com Archaeology & Fossils)

A letter in the Basel papyrus collection describes day-to-day family matters, and yet is unique in its own way: It provides valuable insights into the world of the first Christians in the Roman Empire, which is not recorded in any other historical source. The letter has been dated to the 230s AD, and is thus older than all previously known Christian documentary evidence from Roman Egypt.