The Codex Sinaiticus is dated to the second quarter of the 4th century. It is a splendid manuscript of the Holy Scriptures, which originally contained the entire Old and New Testaments, plus the Shepherd of Hermas, and the Epistle of Barnabas. It is written four columns to the page, in a clear and regular script. The Codex Sinaiticus contains the oldest surviving complete New Testament. The Codex Sinaiticus and the Codex Vaticanus are the oldest copies of the scriptures written on parchment. Scholars feel that they preserve a very early level of the text, and their study is absolutely essential for anyone wishing to study the history of the text of the scriptures.

The Codex Sinaiticus was described by visitors to Sinai in the eighteenth century, and it was studied extensively by Archimandrite Porphiry Ouspensky. However, the German scholar Constantine Tischendorf was the first to fully appreciate the antiquity and the significance of this manuscript. On his first visit to the monastery, in 1844, he managed to take forty-three folios of the codex with him. These are preserved to this day in the University of Leipzig. On his third visit to the monastery, in 1859, he asked that the codex be sent to Cairo so that he could make a transcription of it for publication. It was in doing so that he realized the complexity of the text, and so he asked that he be allowed to take the manuscript to Russia, where he would have it on hand when he made a printed edition of the text. He promised to return the manuscript to the monastery at its first request.

Instead, the manuscript was kept by the Russian government, and then sold to England in 1933 for the price of £100,000, a huge sum of money for its time. It is now one of the great treasures of the British Library. The Russians retained eight fragments, which remain in the State Library at St. Petersburg.

Tischendorf justified his taking the manuscript, writing that the precious codex was on the verge of being burned by the monks. The monastery has always protested against this slander, and laments the loss of this manuscript.