Tischendorf exemplified the buccaneer image of 19th century archaeology in his pursuit of unknown manuscripts. Alongside his industry in collecting and collating manuscripts, Tischendorf pursued a constant course of editorial labours, mainly on the New Testament, until he was broken down by overwork in 1873.
After a journey through southern Germany and Switzerland, and a visit to Strassburg, he returned to Leipzig, and set to work upon a critical study of the New Testament text. In 1840 he qualified as university lecturer in theology with a dissertation on the recensions of the New Testament text — the main part of which reappeared the following year in the prolegomena to his first edition of the Greek New Testament. His critical apparatus included variant readings from earlier scholars — Elsevier, Knapp, Scholz, and as recent as Lachmann — whereby his researches were emboldened to depart from the received text as used in churches.
These early textual studies convinced him of the absolute necessity of new and more exact collations of manuscripts. From October 1840 until January 1843 he was in Paris, busy with the treasures of the Bibliothèque Nationale, eking out his scanty means by making collations for other scholars, and producing for the publisher, Firmin Didot, several editions of the Greek New Testament — one of them exhibiting the form of the text corresponding most closely to the Vulgate. His second edition retracted the more precarious readings of the first, and included a statement of critical principles that is a landmark for evolving critical studies of Biblical texts.
From Paris, he had paid short visits to the Netherlands (1841) and England (1842). In 1843 he visited Italy, and after a stay of thirteen months, went on to Egypt, Sinai, and the Levant, returning by Vienna and Munich. In 1844, he paid his first visit to the convent of Saint Catherine's Monastery, on Mount Sinai, where he found, in a trash pile, forty-four pages of what was the then oldest known copy of the Septuagint. He deposited them at the University of Leipzig, under the title of the Codex Frederico-Augustanus, a name given in honour of his patron, Frederick Augustus II of Saxony, king of Saxony. The fragments were published in 1846 although he kept the place of discovery a secret.
A great triumph of these laborious months was the decipherment of the palimpsest Codex Ephraemi Syri Rescriptus, of which the New Testament part was printed before he left Paris, and the Old Testament in 1845. His success in dealing with a manuscript that, having been rewritten with other works of Ephrem the Syrian, had been mostly illegible to earlier collators, made him more well known, and gained support for more extended critical expeditions. He now became professor extraordinarius at Leipzig, and married (1845). He also began to publish an account of his travels in the East (2 vols., 1845–46).
In the winter of 1849 appeared the great work now titled Novum Testamentum Graece. Ad antiquos testes recensuit, Apparatum Criticum multis modis canons of criticism, adding examples of their application that are applicable to students today:
Basic rule: "The text is only to be sought from ancient evidence, and especially from Greek manuscripts, but without neglecting the testimonies of versions and fathers."
- "A reading altogether peculiar to one or another ancient document is suspicious; as also is any, even if supported by a class of documents, which seems to evince that it has originated in the revision of a learned man."
- "Readings, however well supported by evidence, are to be rejected, when it is manifest (or very probable) that they have proceeded from the errors of copyists."
- "In parallel passages, whether of the New or Old Testament, especially in the Synoptic Gospels, which ancient copyists continually brought into increased accordance, those testimonies are preferable, in which precise accordance of such parallel passages is not found; unless, indeed, there are important reasons to the contrary."
- "In discrepant readings, that should be preferred which may have given occasion to the rest, or which appears to comprise the elements of the others."
- "Those readings must be maintained which accord with New Testament Greek, or with the particular style of each individual writer."
These were partly the result of the tireless travels he had begun in 1839 in search of unread manuscripts of the New Testament, "to clear up in this way," he wrote, "the history of the sacred text, and to recover if possible the genuine apostolic text which is the foundation of our faith."
In 1850 appeared his edition of the Codex Amiatinus and of the Septuagint version of the Old Testament (7th ed., 1887); in 1852, amongst other works, his edition of the Codex Claromontanus. In 1853 and 1859 he made a second and a third voyage to the East. On the last of these, with the active aid of the Russian government, he at length got access to the remainder of the precious Sinaitic codex, and persuaded the monks to present it to the tsar, at whose cost it was published in 1862 (in four folio volumes). By those ignorant of the details of his discovery of the Codex Sinaiticus, Tischendorf was accused of buying manuscripts from ignorant monastery librarians at low prices. Indeed he was never rich, but he staunchly defended the rights of the monks at St. Catherine's Monastery when he persuaded them eventually to send the manuscript to the tsar. Even so, the monks of Mt. Sinai still display a letter from Tischendorf promising to return the manuscript to them. In 1869 the tsar awarded him the style of "von" Tischendorf as a Russian noble.
In 1853, he made a second trip to the Syrian monastery but made no new discoveries. He returned a third time in January 1859 under the patronage of Czar Alexander II of Russia to find more of the Codex Frederico-Augustanus or similar ancient Biblical texts. On February 4, the last day of his visit, he was shown a text which he recognized as significant — the Codex Sinaiticus — a Greek manuscript of the complete New Testament and parts of the Old Testament dating to the 4th century.
Meanwhile, also in 1859, he had been made professor ordinarius of theology and of Biblical paleography, this latter professorship being specially created for him; and another book of travel, Aus dem heiligen Lande, appeared in 1862. Tischendorf's Eastern journeys were rich enough in other discoveries to merit the highest praise.
Besides his fame as a scholar, he was a friend of both Robert Schumann, with whom he corresponded, and Felix Mendelssohn, who dedicated a song to him. His text critical colleague Samuel Prideaux Tregelles wrote warmly of their mutual interest in textual scholarship. His personal library, purchased after his death, eventually came to the University of Glasgow, where a commemorative exhibition of books from his library was held in 1974.
He died in Leipzig.
The great edition, of which the text and apparatus appeared in 1869 and 1872, was called by himself editio viii; but this number is raised to twenty or twenty-one, if mere reprints from stereotype plates and the minor editions of his great critical texts are included; posthumous prints bring the total to forty-one. Four main recensions of Tischendorf's text may be distinguished, dating respectively from his editions of 1841, 1849, 1859 (ed. vii), and 1869–72 (ed. viii). The edition of 1849 may be regarded as historically the most important, from the mass of new critical material it used; that of 1859 is distinguished from Tischendorf's other editions by coming nearer to the received text; in the eighth edition, the testimony of the Sinaitic manuscript received great (probably too great) weight. The readings of the Vatican manuscript were given with more exactness and certainty than had been possible in the earlier editions, and the editor had also the advantage of using the published labours of his colleague and friend Samuel Prideaux Tregelles.
Of relatively lesser importance was Tischendorf's work on the Greek Old Testament. His edition of the Roman text, with the variants of the Alexandrian manuscript, the Codex Ephraemi, and the Friderico-Augustanus, was of service when it appeared in 1850, but, being stereotyped, was not greatly improved in subsequent issues. Its imperfections, even within the limited field it covers, may be judged by the aid of Eberhard Nestle's appendix to the 6th issue (1880).
Besides this may be mentioned editions of the New Testament apocrypha, De Evangeliorum apocryphorum origine et usu (1851); Acta Apostolorum apocrypha (1851); Evangelia apocrypha (1853; 2nd ed., 1876); Apocalypses apocryphae (1866), and various minor writings, partly of an apologetic character, such as Wann wurden unsere Evangelien verfasst? (1865; 4th ed., 1866), Haben wir den echten Schrifttext der Evangelisten und Apostel? (1873), and Synopsis evangelica (7th ed., 1898).