Technically speaking, the Bible is not the Word of God. The Word of God is not a book. It is Jesus Christ, Himself. John 1:1 says, "In the beginning was the Word, and the Word was with God, and the Word was God." John is speaking of Jesus Christ here.
The Catholic Church (Apostles and their successors) wrote the New Testament from Holy Tradition (Oral Tradition: 2 Thes 2:15). Jesus didn't write a book, He taught orally. They called Him "Rabbi" (Teacher). Most of the Apostles were illiterate.
A good question would be, why does the Protestant Old Testament have 7 fewer books than the Catholic Old Testament? There were actually two versions of the Old Testament floating around when the Bible was compiled into one book in the late fourth century. One was Hebrew and one was Greek. Before Jesus was incarnated, there was a large contingent of Jews living in Alexandria, Egypt, as merchants. Alexandria was a large port city on the Mediterranean Sea. The common language of the Mediterranean countries with regard to commerce and literature, at that time, was Greek. The Jews, being good merchants, worked there for generations and more or less begin forgetting their Hebrew. But, they knew Greek. So, they contacted Israel and asked for a copy of the existing Scriptures to be translated into Greek, which they did. After Jesus came, and rose to heaven, the Apostles and new Christians begin converting Jews to Christianity, using the Old Testament Scriptures. This, of course, upset the Jewish leaders, who then decided to create an official canon (list of books) for their Old Testament. They purposely left out seven books that had previously been there. So, we now had two versions of Scripture. In the late fourth century, the Catholic Church held three councils to determine which of the 300+ books, documents, letters, etc., that were in circulation were worthy of being considered Scripture. Of all of those, they came up with the 27 that almost everyone agrees are the books of the New Testament. They then chose the Greek version of the Old Testament. Bible scholars have studied both versions of the Old Testament and have determined that 80-85% of the direct and indirect references in the New Testament to the Old Testament point to the Greek version. Therefore, we can conclude that the Greek version was the one Jesus and the Apostles used most often.
Interesting side note about the Hebrew version of the Old Testament that most Protestant bibles use: The seven books that were left out covered the last 400 years before Christ's incarnation. So, if you believe that God purposely and arbitrarily didn't want those 400 years covered, then the Hebew version is for you. If you think that God was consistent in His revelation in the Old Testament and wanted all those years covered, then the Septuagint version is the one you want. That's the version the Catholic Church uses.
