"God's existence is proven by scripture."

Many religions have certain holy books that are revered as true accounts. These are called scripture, text considered sacred and either inspired or directly dictated by a deity.  Many of the adherents to these religions claim that their scriptures prove the existence of their deity. The Bible and Quran are held up by many believers as both guides for belief and historical accounts of reality. Many believers claim that their holy book of choice is uniquely perfect, thus suggesting its divine origin.

This argument presupposes its premise: the people who hold up their holy scripture as evidence are the same people who already believe its contents to be true. In doing so, it falls into the fallacy of begging the question (a form of circular reasoning), where an argument’s question presupposes its answer ⁽¹⁾. This is no different than saying, "This is true because I believe it," which hardly counts as evidence.

Documents Are Not Self-authenticating

Just because something is written in a book does not mean that it's true. This is obvious. There are millions of fictional stories throughout history and plenty of other books that claim to be factual but have been proven to be false. The existence of scripture does not automatically prove anything about the veracity of what those scriptures contain.

Additionally, the scriptures themselves are rife with contradictions. Ultimately, they are books that were written by fallible humans, and though there may be some grains of historical truth within them, there is also ample hyperbole, speculation and mythology.

Scripture Is Often Inconsistent and Inaccurate

Every holy book is full of internal errors, inconsistencies and differing accounts. This makes sense when you consider that these books were pieced together by multiple authors over a span of centuries. If scripture was a document describing historical reality, the basic facts should be consistent from one account to the next.

Some biblical errors are inconsistent with the observable laws of the universe. For example, Genesis 1:1-19 states that God created the heavens and the earth on the first day of creation; the stars, sun, moon and other planets were all created on the fourth day, a full day after the creation of seed-bearing plants. This order makes no sense, as plants require sunlight to grow, even if you ignore the scientific fact that the sun and stars existed long before the earth and flowering plants.

In the Quran, several scientific errors are also apparent. For example, the Quran suggests that the earth is flat with the sun rising and setting in particular parts of the earth (18:86). Such errors make sense when considering the scientific knowledge at the time the Quran was written, but they would not make sense if the Quran had been written by an all-knowing deity, as is believed by Muslims, who hold true the belief that the Quran is the direct and exact word of God told to Mohammed.

Other scriptural problems are internal contradictions. For example, the resurrection story — arguably the single most important event in the Bible from a Christian perspective — is told in a number of different ways. Here are just a few of the inconsistencies between those four versions:

• In Matthew, Jesus was buried by Joseph of Arimathea (Matthew 27:57-60). In Acts, he was buried by a different group of people (Acts 13:27-29).
• Matthew (28:2-5) and Mark (16:5) report that the women at Christ's tomb saw one person or angel. Luke (24:4) and John (20:12) say there were two.
• Mark states that Jesus died the day after Passover meal (Mark 14 - 15). John places the event on the day before the Passover meal (John 18 - 19).

When the scripture can't even come to a consensus about a simple fact like the date of Jesus's crucifixion, it's difficult to accept the accounts as being historically accurate, much less divinely inspired.

Muslims are quick to point out the supposed perfection of their holy book, the Quran. According to many Muslims, the Quran contains foreknowledge of science that predicts modern inventions and discoveries. These claims are dubious ad hoc arguments: Modern-day believers attribute these explanations to the text after the fact. If the Quran actually contained scientific breakthroughs, many of the countless believers who had studied the Quran would have made these discoveries before the scientists. That none of these scientific predictions were revealed by interpretations of the Quran until after they'd come to light by scientists makes such claims highly dubious.

Religious Texts are Man-Made and Fallible

There's a simple explanation for the errors in the Quran and Bible: these documents were written by humans, and in many cases, were stitched together from oral traditions and transcribed decades or even centuries after the events described. Bear in mind, also, that the books of the Bible are largely anonymous. Names like Matthew, Mark, Luke and John were added after the fact by editors and scribes. The actual identity of these authors is unknown ⁽²⁾.

Biblical scholars estimate that the oldest books of the New Testament, Paul's letters, were written around 20 years after the date of Christ's supposed resurrection. Paul was not present for any of the events described in the gospels, and he did not know Jesus personally. The gospels themselves were written even later, between 30 and 70 years after the alleged death of Jesus ⁽²⁾.

Jesus's contemporaries were Aramaic-speaking, illiterate commoners. They could neither read nor write, so stories were passed around orally. Like all gossip, these oral histories are bound to have transformed over time by gaining embellishments, mixing up details and forgetting important facts. Just like any other legend, from the invasion of Troy to the tales of Paul Bunyan, these stories likely contain much more poetic license than actual history.

References:

1. Bennett, Bo. "Begging the Question." Logically Fallacious: The Ultimate Collection of over 300 Logical Fallacies. EBookIt.com, 2012.
2. Ehrman, Bart D. Jesus, Interrupted: Revealing the Hidden Contradictions in the Bible (And Why We Don't Know About Them). New York: HarperOne, 2009.