It depends on who you ask, as most religions claim a different answer, but most historians believe it was written in Aramaic, the most common language spoken in that time period.

Hebrew, Aramaic, and Greek. That's why the first part of the bible is often referred to as the Hebrew Scriptures and the later part as the Greek Scriptures since those are the languages that they were mainly written in at the time of writing.

Some particular forms of Hebrew, Aramaic and Greek. "Biblical languages are any of the languages employed in the original writings of the Bible. Partially owing to the significance of the Bible in society, Biblical languages are studied more widely than many other dead languages. Furthermore, some debates exist as to which language is the original language of a particular passage, and about whether a term has been properly translated from an ancient language into modern editions of the Bible. - Language of the Hebrew Bible: The Hebrew Bible (Hebrew: תנ"ך), also known as the Tanakh, consists of 39 books. "Hebrew" in "Hebrew Bible" may refer to either the Hebrew language or to the Hebrew people who historically used Hebrew as a spoken language, and have continuously used the language in prayer and study, or both. The texts were mainly written in biblical Hebrew, with some portions (notably in Daniel and Ezra) in Aramaic. Biblical Hebrew, sometimes called Classical Hebrew, is an archaic form of the Hebrew language. The very first translation of the Hebrew Bible was into Greek. This is known as the Septuagint (LXX), which later became the received text of the Old Testament in the church and the basis of its canon. This began sometime in the 2nd or 3rd century BCE, with the first portion of the Hebrew Bible, the Torah, being translated into Koine Greek. Over the next century, other books were translated (or composed) as well. This translation became known as the Septuagint and was widely used by Greek-speaking Jews, and later by Christians. It differs somewhat from the later standardized Hebrew (Masoretic Text). This translation was promoted by way of a legend that seventy separate translators all produced identical texts. The Latin Vulgate by Jerome was based upon the Hebrew for those books of the Bible preserved in the Jewish canon (as reflected in the Masoretic Text), and on the Greek text for the rest. Other ancient Jewish translations, such as the Aramaic Targums, conform closely to the Masoretic Text, and all medieval and modern Jewish translations are based upon the same. Christian translations also tend to be based upon the Hebrew, though some denominations prefer the Septuagint (or may cite variant readings from both). Bible translations incorporating modern textual criticism usually begin with the Masoretic Text, but also take into account possible variants from all available ancient versions. - Languages of the New Testament: The books of the Christian New Testament are widely agreed to have originally been written in Greek, specifically Koine Greek, even though some authors often included translations from Hebrew and Aramaic texts. Certainly the Pauline Epistles were written in Greek for Greek-speaking audiences. See Greek primacy for further details. Koine Greek was the popular form of Greek which emerged in post-classical antiquity (c.300 BC – AD 300), and marks the third period in the history of the Greek language. It is also called Alexandrian, Hellenistic, Common, or New Testament Greek. Some scholars believe that some books of the Greek New Testament (in particular, the Gospel of Matthew) are actually translations of a Hebrew or Aramaic original. A famous example of this is the the opening to the Gospel of John, which some scholars argue to be a Greek translation of an Aramaic hymn. Of these, a small number accept the Syriac Peshitta as representative of the original. See Aramaic primacy. However, the received text of the New Testament is Greek, and nearly all translations are based upon the Greek text." Source and further information: http://en.wikipedia.org/wiki/Bible_language

This answer is part of a comment thread I have with 1000 Horse whatever his name is. I couldn't fit it in the comment thread so to continue.... The Vedas are four in number -- the Rig, Sama, Yajur and Atharva. Each Veda itself is composed of parts, the Samhita (the mantras in verse), the Brahmana (rituals and liturgy in prose) and the Aranyakas and Upanishads ( the philosophical works). The layers of text reflect the chronology of composition, with the Samhita being the most ancient part and the Upanishads the most recent. It is agreed among scholars that the Rigveda Samhita (RV for brevity) is by far the oldest part of the Vedas. So it is to that we, as history buffs, shall turn our attention. Tradition has it that the Vedas are eternal or ageless scriptures, "heard" by ancient seers and collected by them. Veda Vyasa, the author of the Mahabharata, is supposed to have arranged the Veda into the form that we know today. The traditional date for Veda Vyasa is around 3000 BC. The Vedas are thus ahistorical; modern Arya Samajis continue to hold the belief that the RV, being eternal, cannot hold anything so trivial as historical information. From a more modern point of view, the date of the RV is less certain. Scholars do agree that the RV is probably the oldest source material for Indian and even Indo-European history, but do not have a firm date ( The languages Greek, Latin, Sanskrit, etc., show so many similarities in grammar and vocabulary, and show systematic shifts in pronunciations that it is clear that they all belong to a common family tree of languages, called the Indo-European languages. The RV is probably the most ancient sample of an Indo-European language that we have). The RV contains 1028 suktas (hymns) with a total of 10552 mantras (verses) arranged in 10 Mandalas or books. This is extremely large compared to other ancient texts that have survived. For example, what survives of the Avesta, the sacred text of Zoroastrianism (the religion of the Parsis), is about one-tenth or so of the RV. The Avesta is written in a language very closely related to the RV, and has many features in common with the RV, that untangling the relationship between the Avesta and the RV is vital to the understanding of ancient Indian (and Iranian ) history. The conventional dates for the RV in modern scholarship place the RV between 1700 BC and 1000 BC. An example of how these dates are constructed can be found on the Indology list. Some of the argument is reproduced here. The model in which these dates are constructed is one in which the Aryans, speakers of Vedic Sanskrit or a precursor language, entered India sometime after 1700 BC, that is, after the end of the urban period of the Harappan culture. The Aryans entered India as invaders or as peaceful migrants. The Aryans brought along the horse and the horse-drawn chariot, which gave them a military advantage over the original inhabitants, if they were invaders; and items of high prestige if they were migrants. Either way the Aryans became the elite, and were able to impose their language and religion on the people. They may have invented the castes to perpetuate their hegemony. It was these people who composed the RV. The Mandalas of the RV can be arranged in an order (believed to be chronological ) with a shift in the names of rivers and places from the older Mandalas to the newer ones showing a movement of the Aryans from Afghanistan and Northwest India into Punjab and ultimately into the Gangetic plains. The culture which we glimpse in the RV does not know of iron, and so the RV can be no later than the beginning of widespread use of iron in India (around 1000 BC to 900 BC). The RV can be no older than the first Aryan-style chariots, first recorded around the Ural mountains on the Asia-Europe border around 2000 BC. The RV is "horse-obsessed" and so can be no older than the earliest domesticated horses in India, which date is said to be later than 1700 BC. The Harappan culture had no horses, insists Western scholarship. Various putative horse remains found around Harappan cities either have been carelessly excavated so that reliable dates cannot be determined, and are probably post-1700 BC or the bones are not those of the horse (Equus Caballus) but rather are of the wild Kutchi onager (Equus hemiones khur). Horses are not found on the famous Harappan seals and the few figurines found of horse-like animals are said to be representations of the onager. Some few words found in the cuneiform-inscribed clay tablets of the Middle East show a borrowing from a language that is said to be pre-Vedic Sanskrit. Since these writings can be reliably dated to around 1400 BC, the RV is said to be most likely later than that date. Finally, the RV is believed, in this model, to have been composed in a relatively short period of time; perhaps a century ...it goes on and on but you get the idea.