At first glance, it appears that the answer to the question of what are Christianity's sacred texts is straightforward. There is only one sacred text, the Bible and it contain only two major parts, the Old Testament and the New Testament. Upon a deeper inspection, however, the matter is not so straightforward. Not only do different divisions of Christianity put different books into the two parts, but it took several centuries for any sort of regularization and general agreement about which books should belong to them.
Early Christianity quickly developed the belief that it replaced the people Israel (i.e., the Jews) as God's chosen people. Paul wrote that God cut the branch of Israel off the tree and grafted on the new branch of Christianity (see Romans 11). Thus Israel represented the old covenant with God and the Christians took part in a new covenant with God. The Old Testament was thus seen as the books that described the old covenant and prophesied the new covenant. The New Testament contained the texts that laid out the key elements of the new covenant.
In the Eastern Mediterranean, the Orthodox Church kept the Bible in Greek, although over the centuries, the Orthodox churches of different countries translated books into their own language. The Greek translation was known as the Septuagint. In the West, the Catholic Church translated the Bible into Latin (done by Jerome in the fifth century). This translation became known as the Vulgate, and was the official Bible of Catholicism until the changes of Vatican II in the 1960's. The Protestant Churches made their own translations starting in the Reformation. Martin Luther translated the scriptures into German and the Geneva Bible (1560) was popular among English-speaking Protestants. In 1611, the King James translation was published, so-called because it received the royal patronage of James I of England. Of course, the Bible has been translated into most of the languages of the world. In the early Christian centuries, it was translated into Coptic, Armenian, Arabic, and Syriac--to name just a few. In the modern world, missionaries have translated it into nearly every written language in existence. Languages such as English, have had the Bible translated dozens of times in the past century.
It can be said that the Christian Church borrowed the Old Testament from Judaism. But this simple statement raised two issues for the early church. First, did there need to be an Old Testament? Since its books were about the old covenant and Christianity was about the new covenant, why did the Christianity need to know about the old covenant? Many important early Christian thinkers held that there was no need to have an Old Testament; people like Marcion actually argued that Christianity did not need to know about the Jews, since Christianity believed that God had rejected them and replaced them with the Christians. But the dominant position was the reply that Christianity needed to know about God's work on earth prior to Jesus, and that information was contained in the Old Testament.
Second, the other important issue was the question of which books should be in the Old Testament. The easy answer is that since the Old Testament is about the Jews, then Christianity should borrow the sacred books of Judaism. But which sacred books? The answer was: the books written in Greek. Although the earliest Christians presumably were from the Land of Israel and spoke Aramaic and/or Hebrew, by the time this question was raised, most Christians were from the Greek-speaking countries of the eastern Mediterranean. They read their books in Greek. So they used the Jewish books that had been translated into or originally written in Greek. These included the books of the Torah, the Prophets and the Writings (which were mostly written originally in Hebrew), but also different texts that came to be known as the Apocrypha. These included works such as: Tobit, 1st and 2nd Maccabees, the Wisdom of Solomon, Ecclesiasticus, Judith, additions to the book of Daniel and Esther, Baruch, the Letter of Jeremiah, and 1st and 2nd Esdras. In the Western Church (i.e., Catholic), these texts were all included in the Old Testament, as decided by the councils of Hippo and Carthage in the late fourth century. In the Eastern Church (i.e., the Orthodox Church), these texts were included, along with 3rd and 4th Maccabees, Psalm 151, and the Prayer of Manasseh. Other churches, such as the Ethiopian Orthodox Church, included other texts, such as 1st Enoch.
These collections remained fairly standard until the Protestant Reformation in sixteenth-century Europe. Then, the protestant churches distinguished themselves from the Catholic Church by revising the canon of the Old Testament. They rejected all the texts that were not in the Jewish Bible, the Tanak. Thus all the books of the Aprocrypha were taken out.
It took several centuries for a canon of the New Testament to be agreed upon. Part of the problem was that during the first few centuries, there was not unified Christianity, but instead many different versions and theologies of Christianity in cities across the Mediterranean world. (For more information, see the Organizations page.) During the first few centuries, there were at least a couple dozen gospels written, as well as Tatian's Diatessaron, a compilation of the four gospels reworked into a single narrative. Clement of Alexandria, a second-century theologian, used the Gospel of the Hebrews, the Gospel of the Egyptians, the Letter of Barnabas, and the Didache.
In the fourth century, the Bishop Athanasius set out a definitive proposal for identifying the canonical books of the New Testament, which was pretty much accepted by all forms of Christianity over the next few centuries. This was the four gospels (Matthew, Mark, Luke and John), Acts, the 14 letters of Paul (including Hebrews), letters by the other early apostles such as John, Peter and Jude, and Revelation. Even with this agreement, New Testament manuscripts from the following century or two still sometimes include 1st and 2nd Clement, the Letter of Barnabas and the Shepherd of Hermas.
Of the books that became part of the New Testament, the oldest are the letters of Paul, usually considered to have been written in the 40s and 50s of the first century. Other letters are thought to have been written over the next couple of decades. Of the four gospels, Mark is the earliest at about 68-70 ce, while John is the latest at about 110. Acts is later than Luke (around 100) and Revelation was probably composed in the 90s.
Once finalized, the canon of the New Testament remained fairly stable. The division between the Catholic Church in the West and the Orthodox Church in the East had no impact on the books included, and neither did the Protestant Reformation in the sixteenth century.