Question: "What is the Nag Hammadi library?"
Answer: Nag Hammadi is a town in northern Egypt where a collection of ancient writings was discovered in 1945. The collection of writings has since been titled the Nag Hammadi library, or the Nag Hammadi scrolls, or the Nag Hammadi codices. The vast majority of the scrolls in the Nag Hammadi library represent the writings of what was/is known as Christian Gnosticism.
The Nag Hammadi library is frequently pointed to as an example of "lost books of the Bible." According to the conspiracy theory, the early Christians tried to destroy these Gnostic writings because they contained secret teachings about Jesus and Christianity. The Nag Hammadi library was supposedly the result of faithful efforts of Gnostic monks to save the truth about Jesus Christ from the persecution of non-Gnostic Christians. The Nag Hammadi scrolls include works known as the gospel of Truth, the gospel of Philip, the apocryphon of John, the apocalypse of Adam, and the acts of Peter and the Twelve Apostles. The most famous Nag Hammadi scroll is the only known complete copy of the gospel of Thomas.
So, what are we to make of the Nag Hammadi library? Should some or all of the scrolls be in the Bible? Absolutely not. First, the Nag Hammadi scrolls are forgeries. The Apostle Philip did not write the gospel of Philip. The Apostle Peter did not write the acts of Peter. The gospel of Thomas was not written by the Apostle Thomas. These scrolls were fraudulently written in their names in order to give them legitimacy in the early church. Thankfully, the early church fathers were nearly unanimous in recognizing these Gnostic scrolls as fraudulent forgeries that espouse false doctrines about Jesus Christ, salvation, God, and every other crucial Christian truth. There are countless contradictions between the Nag Hammadi library and the Bible.
While the Nag Hammadi library was an exciting find, the only "value" in the Nag Hammadi library is that the scrolls give us insight into what early "heretics" taught and practiced. Recognizing the false doctrine that plagued the early church will help us better to understand it and refute it today.