The "critical text" of the Greek New Testament is the term often used (especially in King James Only literature) for the Greek text found in most modern printed editions of the Greek New Testament, such as the Nestle-Aland Greek New Testament or that of the United Bible Society. The text is not called "critical" in the sense of "criticizing" or being "critical of" the text (i.e., challenging, insulting, or questioning the text). It is called "critical" because it is the result of a process known as "textual criticism," which is the science of comparing the different manuscripts and other witnesses to an ancient text of which we lack the original copy in an effort to determine the precise wording of the original. Editions of the modern critical text of the Greek New Testament are not only the result of Textual criticism, they also contain tools to aid in textual criticism, such as a "textual apparatus" or a list of all the other major variants in a given verse and which witnesses attest to each option. Thus, while the publishers choose which readings they think are most likely and place them in the main body of the text, the student or scholar using the volume is given all the other options in the notes so that they can understand the publisher's decision and decide whether or not they agree with it. Thus, while modern translations are almost all based on the critical text, the translation committees will in some places deviate from from that text when they are convinced one of the marginal readings is more likely to be original. This is the reason for some of the minor differences between modern translations.
A Brief History of the Modern Critical Text
The New Testament documents were originally written in Greek. Early on, most Christians spoke Greek (even if as a second language). Even the Old Testament was read in the Greek Translation called the Septuagint. In time, however, as Christianity spread and cultures changed, a smaller and smaller percentage of Christians understood Greek, and in many regions, none did at all. Thus, translations became more and more central to Christian life. In Western Europe, where Latin was the dominant language, Greek was all but forgotten for many centuries as churches began to rely fully on the Bible translation known as the Latin Vulgate. Even as Latin itself became alien to many of the poorer masses, it was still the language of scholarship and of the educated, literate class. A professor in Germany could correspond with another in England or France without having to learn one another's native language because they both wrote in Latin. Such a situation maintained the status quo for some time. Eventually, however, Greek manuscripts began to come into the hands of western scholars, and an interest arose in getting back to the original text.
Some scholars questioned whether the Vulgate had been accurately translated from the Greek. Others, not quite so bold, held the Vulgate in the highest honor, but questioned whether or not it had been copied correctly over the years. Were the copies they had in the late middle ages still the original Vulgate that Jerome translated in the 4th century AD? All of these questions led to a resurgence of critical examination of Greek and Latin manuscripts. The invention of the printing press opened great opportunities for such studies, and one such scholar, a Dutch priest named Desiderius Erasmus, published the first Printed Greek New Testament in 1516 paired with a fresh Latin translation. He produced his Greek text primarily by comparing the small number of incomplete manuscripts he had available to him to piece together the full Greek text. In a few places, none of his manuscripts contained a certain passage, and so Erasmus was forced to translate from the Vulgate back into Greek to fill in the blanks. It was far from a perfect work, and one that Erasmus would repeatedly revise over five editions, but it was the first effort at a critical Greek New Testament in the modern world, and thus was a ground breaking achievement.
After Erasmus, a man named Robert Estienne (more often known today by his Latin name, Stephanus) continued the work, producing four more editions of his own. Stephanus used Erasmus' text as his base, and made edits based on the examination of further manuscripts. His 1550 edition was especially influential. In it, he added marginal notes with alternate readings found in some 14 additional manuscripts (as well as those found in a different printed Greek text called the Complutensian Polyglot). Thus, Stephanus' 1550 edition is called by some the first truly critical text. After Stephanus, a Genevan scholar named Theodore Beza continued the work even further, making additional edits over the course of nine more editions. The editions of Erasmus, Stephanus, and Beza form the basis of the tradition later known as the "Textus Receptus," (or "TR"). They also mark the beginning of the critical text.
After the time of Beza, a number of attempts were made to further advance the critical text tradition. One significant early example was that of an English clergyman named John Mill, who published a Critical Greek New Testament in 1707. Mill used the 1550 Stephanus text as his base text, but added notes on variant readings from about 100 manuscripts. Shortly afterward, Ludolf Küster published a revision of Mill's Greek New Testament in 1710 with added notes from 12 additional manuscripts. While the base text in these editions was unchanged, they provided scholars and translators with far greater knowledge of the kinds of variants that existed among the known manuscripts. While 112 manuscripts is extremely small compared to the thousands we know today, this was a huge step forward in critical scholarship.
In 1831, a man named Karl Lachmann published perhaps the first Critical Greek New Testament whose main text did not align with the TR tradition. Rather than simply making minor edits to the work of Erasmus, Stephanus, and Beza, Lachman decided to start fresh, examining the manuscripts and developing his own critical text based on what he found there. He arrived at his text through comparing early and diverse manuscripts, with special emphasis on recently discovered manuscripts that were far more ancient than those used in producing the TR.
Constantine von Tischendorf, a scholar most famous for his discovery and acquisition of a 4th-century Greek Bible that came to be known as Codex Sinaiticus, also released released a critical edition of the Greek New Testament in 1849. It brought a unique addition to the field in that it contained a set of "canons" or rules for how textual variants were evaluated, thus informing the reader exactly what method he had used to arrive at his readings when the manuscripts differed. A textual apparatus (noting specific variants in other manuscripts) was added in later editions of the work. Samuel Prideaux Tregelles similarly published a critical edition of the New Testament, which he had worked on for years while traveling through Europe to study and document a wide variety of manuscripts. He published his work in installments between 1857-1872. Many other such men could also be noted.
The most famous (or, to some, infamous), however, of the 19th-century critical editions of the Greek New Testament was that of Brooke Foss Wescott and Fenton John Anthony Hort. The Wescott-Hort text was published in 1881. Its particular fame is related to the fact that it came by far the closest to the text that modern textual scholars generally agree upon today. Indeed, the similarity has often provoked erroneous and exaggerated claims from critics that modern critical texts are merely reprintings of the Wescott-Hort text. This is untrue. Modern texts differ from Wescott and Hort in a number of places. Manuscript discoveries over the last hundred years have made a huge impact, and modern scholars freely point out that Wescott and Hort often gave exaggerated weight to certain manuscripts against other considerations. Still, it is certainly true that Wescott and Hort came notably closer to the conclusions that later textual scholars would arrive at than any of the other 19th-century attempts at a critical text. Indeed, they helped shape the methodology that many textual critics would use for many years to come.
In 1898, Eberhand Nestle published the first edition of the Novum Testamentum Graece. This was a Greek New Testament based on combining the readings from Tischendorf's text, the Westcot-Hort text, and others into one critical volume. It was not an attempt to build on just one of the previous critical texts, but rather to draw out the best in all of them (primarily through siding with the majority reading among the various critical texts). In later editions, he consulted further texts and began adding an apparatus noting variant readings from specific manuscripts. After his death, his son Erwin continued his work, improving the base text and expanding the apparatus over several more editions.
In the 20th century, a host of important new manuscripts continued to be discovered. In 1952, Kurt Aland joined the project as an editor of the 21st edition. Over the next several editions, Aland drew from newly discovered papyri and other early manuscripts both to improve on the base text and to expand the apparatus. By the 26th edition, the "Nestle-Aland" text had been significantly updated from the composite Nestle had originally produced to a text derived directly from data on thousands of known manuscripts. The United Bible Society (UBS) had also begun printing Greek New Testaments, which came to rely on the same base text as the Novum Testamentum Graece (the Nestle-Aland text.) Both the UBS and the Nestle-Aland texts (which use the same base text but differ somewhat in the scope and detail of the information they provide in the apparatus) have become the standard Critical Greek Texts of the day. When people speak of the "modern critical text," they generally have the UBS/NA platform in mind.