Teabing begins Sophie's education with Leonardo Da Vinci's opinion on the New Testament, which is…not positive.
Da Vinci, and Teabing, believe that the Bible is highly fallible, as it is above anything else, a product of men.
While Jesus Christ was "a historical figure of staggering influence", the things that were written about him in the Bible had biases and political motivations.
He informs Sophie that there were over eighty gospels considered for the New Testament, but the ones that made the final cut were chosen by the pagan Roman emperor Constantine the Great.
Time for a history lesson, courtesy of Sir Leigh Teabing.
Constantine was the head priest of Rome's official religion at the time of his reign, which was sun worship, or Sol Invictus (the Invincible Sun).
Unfortunately, a religious war was on the cusp of breaking out between Christians and pagans, so in 325 A.D. Emperor Constantine chose Christianity as the official religion to unify Rome.
In doing so, he converted sun-worshipping pagans to Christians by fusing pagan symbols, dates, and rituals into the still-growing Christian traditions, essentially melding them into a new religion everyone would be happy with.
So, in order to strengthen this new version of Christianity, Constantine held the Council of Nicaea, which most importantly to our story, established the divinity of Jesus.
This made him above reproach from pagan challenges, but also made it so that people had to bow to the new Roman Catholic Church—and the Vatican—in order to redeem themselves spiritually.
In order to reinforce Jesus's godlike traits over his mortal ones, Constantine destroyed the gospels that talked about him like a normal man, and commissioned a new Bible that only told of his divinity.
Some of these gospels survived, like the Dead Sea Scrolls, and the Coptic Scrolls, which tell a story of a much more human Jesus Christ.
The Vatican tried to suppress those writings—not out of some evil sense of duty—but because they go against their sincere views of Christ, so they think they could therefore be only false testimony.
After the history lesson, Teabing offers to show Sophie a painting Da Vinci did of the Grail, but first he makes her look at a picture of The Last Supper.
The fresco is supposed to show the definite arrival of the Holy Grail, where Jesus drank from the chalice and passed it, creating the first communion.
But everyone at the table has cups. No chalice. What the—?
Teabing finally comes out with it: The Holy Grail isn't a thing. It's a person.