An opening from one of the two bound volumes at The British Library. What is Codex Sinaiticus? Codex Sinaiticus, a manuscript of the Christian Bible written in the middle of the fourth century, contains the earliest complete copy of the Christian New Testament. The hand-written text is in Greek. The New Testament appears in the original vernacular language (koine) and the Old Testament in the version, known as the Septuagint, that was adopted by early Greek-speaking Christians. In the Codex, the text of both the Septuagint and the New Testament has been heavily annotated by a series of early correctors.