Because the books of the New Testament were all written in the late first century, it is easy to assume that the Christian Bible took the form it has now by around the year 100. In fact, in took another three centuries.
The word Bible comes from the Greek, just meaning "book." Judaism, Christianity, and Islam are all "religions of the book," meaning that there is a specific book for each of them which is assumed to contain the core of their religious teachings. Other religions certainly have holy writings, but not a single, definitive book.
The majority of the Christian Bible is made up of books from the Hebrew Bible, now called the Old Testament. These books were written in Hebrew over close to a thousand years, combining laws, the history of the Jewish people, songs, sayings, and stories. Different groups included slightly different collections and often arranged them differently. Especially after the Romans razed the Temple in Jerusalem in the year 70 and drove a number of Jews out of the Holy Land, including that sect of Judaism that became Christianity, it was easy for these groups to lose track of each other.
For the purposes of the Christian Bible, one of the most important of these groups of Jews had had highly-educated, Greek-speaking religious leaders back in the second century BC, who translated their Hebrew Bible into Greek. This was called the Septuagint, because the (highly unlikely) story was that seventy learned translators (Septuagint just means 70) sat down independently, and all miraculously translated the Hebrew Bible into Greek exactly the same, word-for-word.
Early Christian leaders and writers were all Greek-speaking; it was the learned language of the eastern Mediterranean. The books of the New Testament were all written in Greek, and early Christians used the Septuagint version of what eventually became known as the Old Testament.
However, the Bible took a while to take shape. The four Gospels, Matthew, Mark, Luke, and John, were four separate stories of the life of Jesus, each one explaining what Jesus really meant in their own terms—and implying that the others had it wrong. Quite early, with no way to choose between them, Christians just lined them all up back-to-back and hoped for the best. Some other writings, like the Gospel of Thomas--which in spite of its title was just a book of sayings--or stories about Jesus's childhood drifted in and out of early collections.
Even more problematic was the Old Testament. From the second century on, Christians jettisoned huge chunks of Jewish law, giving up for example the dietary restrictions, male circumcision, limits on travel on the Sabbath, and even indeed celebrating the Sabbath on Saturday, moving it to Sunday. Some thinkers suggested that since so much to it was seen as superseded, it really wasn't necessary.
But the decision to include the Old Testament in the Christian Bible was a reaction to an early heresy, which argued that God had written the books of the New Testament, but the devil had written the books of the Old. A council (important church decisions were always made in council) decided that No, God had written the whole thing, which meant that it really did belong.
As Christianity became widely adopted in the Roman Empire, it was considered a problem that it was in Greek. The western Mediterranean and Rome's conquered territories were Latin-speaking. So around the year 400 Jerome translated the whole thing from Greek into Latin, using the Septuagint (Greek) version of the Old Testament. The purpose was to make the Bible accessible to everybody. This Latin Bible, still the basis of the modern Catholic Bible, is known as the Vulgate, because ordinary (vulgar) people could read it. It is ironic that it took until the 1960s for Catholics to accept a Bible in something other than Latin--so that people could read it easily, when the original purpose of having it in Latin was exactly so that people could read it easily.
The Protestant Bible is based (as was Jerome's) on the Greek for the New Testament, but it rejected the Septuagint for the Jewish Bible in Hebrew. The Jewish Bible of the sixteenth century was actually somewhat different from the version the Septuagint translators had used 1800 years earlier, so the Protestant Bible is missing a number of Old Testament books still in the Catholic Bible, including Tobit, Ecclesiasticus, the Wisdom of Solomon, and Bel and the Dragon; Protestants collectively call these these apocrypha. Russian Orthodox, Greek Orthodox, Ethiopian, and other smaller sects of Christians all have slightly different collections.
Although in Islam only the Koran (their holy book) in Arabic actually counts as the real Koran, in Christianity the Bible is still the Bible no matter what language it is translated into.
Click here for the next installment, the Bible in the Middle Ages.