Letter of Pliny the Younger.  One of Pliny's letters from 112 C.E. (80 years
after the death of Christ) is the first surviving pagan document to mention Jesus.  Jesus' influence did not become "staggering" until long after his life was over.

“Jesus Christ was a historical figure of staggering influence . . . .   As the prophesied Messiah, Jesus toppled kings, inspired millions, and founded new philosophies. 

With reference to Jesus’ marriage to Mary Magdalene: “It’s a matter of historical record.”

“The point here,” Langdon said, motioning back to the bookshelf, “is that all of these books substantiate the same historical claim.”

“That Jesus was a father.”

“Yes. . . . Mary Magdalene was pregnant at the time of the crucifixion.  For the safety of Christ’s unborn child, she had no choice but to flee the Holy Land.  With the help of Jesus’ trusted uncle, Joseph of Arimethea, Mary Magdalene secretly traveled to France, then known as Gaul.  There she found safe refuge in the Jewish community.  It was here in France that she gave birth to a daughter.  Her name was Sarah.”

Sophie glanced up.  “They actually know the child’s name?”

“Far more than that.  Magdalene’s and Sarah’s lives were scrutinously [sic; author probably means “scrupulously”] chronicled by their Jewish protectors.  Remember that Magdalene’s child belonged to the lineage of Jewish kings—David and Solomon.  For this reason, the Jews in France considered Magdalene sacred royalty and revered her as the progenitor of the royal line of kings.  Countless scholars of that era chronicled Mary Magdalene’s days in France, including the birth of Sarah and the subsequent family tree.”

Brown's claims about the vast number of books
that "substantiate" the marriage of Jesus to Mary
Magdalene and their subsequent child all really
boil down to this one source.