Codex Sinaiticus (London, Brit. Libr., Add. 43725; Gregory-Aland no. א (Aleph) or 01) is a complete, 4th century uncial manuscript of the New Testament. It also contains substantial portions of the Septuagint. Along with Codex Vaticanus, Codex Sinaiticus is one of the most valuable manuscripts for Textual criticism of the Greek New Testament, as well as the Septuagint.

Codex Sinaiticus was found by Constantin von Tischendorf on his third visit to the Monastery of Saint Catherine, on Mount Sinai in Egypt, in 1859. The first two trips had yielded parts of the Old Testament, some from a rubbish bin. The emperor Alexander II of Russia sent him to search for manuscripts, which he was convinced were still to be found in the Sinai monastery. In May 1975 during restoration work, the monks of St. Catherine's monastery at Sinai discovered a room under the St. George chapel which contained many parchment fragments. Among these fragments, thirteen missing pages from the Sinaiticus Old Testament were found.

The story of how von Tischendorf found the manuscript, which contained most of the Old Testament and all of the New Testament, has all the interest of a romance. Von Tischendorf reached the monastery on January 31; but his inquiries appeared to be fruitless. On February 4, he had resolved to return home without having gained his object. "On that day, when walking with the provisor of the convent, he spoke with much regret of his ill-success. Returning from their promenade, Tischendorf accompanied the monk to his room, and there had displayed to him what his companion called a copy of the Septuagint, which he, the ghostly brother, owned. The manuscript was wrapped up in a piece of cloth, and on its being unrolled, to the surprise and delight of the critic the very document presented itself which he had given up all hope of seeing. His object had been to complete the fragmentary Septuagint of 1844, which he had declared to be the most ancient of all Greek codices on vellum that are extant; but he found not only that, but a copy of the Greek New Testament attached, of the same age, and perfectly complete, not wanting a single page or paragraph." This precious fragment, after some negotiations, he obtained possession of, and conveyed it to the Emperor Alexander, who fully appreciated its importance, and caused it to be published as nearly as possible in facsimile, so as to exhibit correctly the ancient handwriting.

The entire codex consists of 346 1/2 folios, written in four columns. Of these 199 belong to the Old Testament and 147 1/2 to the New, along with two other books, the Epistle of Barnabas and part of The Shepherd of Hermas. The books of the New Testament are arranged in this order: the four Gospels, the epistles of Paul, the Acts of the Apostles, the Catholic Epistles , Revelation. Though when parts of Genesis and Book of Numbers were later found in the binding of other books, they were amicably sent to Tischendorf, the Codex is currently regarded by the monastery as having been stolen.

Of its prior history, little is known. The colophons to Esdras and Esther indicated that it had been in Caesarea Palaestina in the 6th or 7th centuries. It is speculated to have been written in Egypt.

The Codex was purchased by the British Library in 1933 from the Soviet Union for £100,000.

The Codex has since been split into four portions now in British Library of London, St. Catherine's Monastery of Sinai, University Library of Leipzig, and the National Library of Russia of St Petersburg.

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