How do we know that the Bible is a unified and complete book in its presentation to us and that it alone contains the written revelation of God for his people?
While the Bible has many human authors through which the text was written, there is one divine author. This is clear by looking at its overall unity. There is not a humanly produced book, wherein multiple authors have contributed over a long span of time, that contains the unity that scripture contains. Not only does the Bible not contradict itself, it also presents a progression of theology that could not have evolved from the imagination of men. Themes and theological concepts are found in their infancy in early Old Testament writings, are developed further in later Old Testament writings, and are found complete within the New Testament—all without contradiction or inconsistency.
More importantly than its unified nature are the many claims that the Bible makes of itself being God’s word. Throughout the scriptures there are commands to “write this down” or “speak this to my people” given by God to his prophets and apostles. The Old Testament itself contains more than 600 instances of “and God said” or “thus says the Lord.” That in itself is an occurrence of about once every 35 verses. The New Testament contains numerous direct quotes from Jesus himself, again being God’s speech recorded by the Apostles. The Bible goes as far as to refer to itself as being the very “breath” of God and thus the revelation of God to his people.
To those who would suggest that there are other texts that necessarily supplement the Bible that also contain God’s word, the Bible contains strong warnings that judgment will come upon those who suggest such things. The Apostle Paul wrote to the Galatians and told them that anyone who proclaimed a gospel not consistent with that of scripture would be accursed. The consistent witness of every prophet and apostle within the history of the Biblical writings is that these words that are recorded in the scriptures contain the very words of God.