Christianity is not a myth
Perhaps the most common argument made today against Christianity is the idea that the Gospels were written many centuries after the fact. This idea that the original story became lost and garbled as it was transmitted across the centuries, each generation embellishing and adding to the grandiosity, until you have a miracle-working Jesus who rose from the dead.
The problem with this theory is that the Gospels were almost certainly written within the first century, during the lifetime of the original eyewitnesses. We have many pieces of evidence that point to this.
(1) The four Gospels display a remarkable knowledge of the local geography of first century Palestine. Keep in mind that ancient Palestine was radically altered after AD 70, when the Romans destroyed Jerusalem and the Jewish people were scattered across the Mediterranean world. Therefore, there was only a short window of time in which people would have had a reliable memory of the historical setting of ancient Jewish Palestine.
The Siege of Jerusalem by David Roberts
The sheer density of geographic markers: names of towns (Sychar), regions (Decapolis), bodies of water (Siloam), distinctive locations (Golgatha) are remarkable, especially when compared to the Gnostic Gospels which have very few geographic markers. The Gnostic Gospels were a kind of fan fiction written centuries after the fact, mostly in Egypt, and lack the telltale signs of authenticity.
Many of the geographic locations were subsequently abandoned or lost after the Jewish-Roman War of AD 70. For example:
John names the pool of Bethesda with its five rows of columns. Scholars doubted the existence of such a pool until it was rediscovered in the 19th century.
The Gospels name Capernaum as the main site from which Jesus conducted ministry in Galilee. This town was later lost and only rediscovered in the 20th century.
According to John, Pilate brought Jesus to a place called the “Stone Pavement” (Gabbatha in Aramaic) for judgment. This structure was uncovered, along with inscriptions, in the 19th century.
(2) The four Gospels display authentic names from first century Palestine. This field of study is called Onomastics, which looks at names and frequency of names. Common names are very specific to a time period and geographic location. They are a unique time marker, as distinctive as a fingerprint.
If you count up all the names in the Gospels, they very accurately match what historians have painstakingly reconstructed based on inscriptions, burial boxes, and documents (like the Dead Sea Scrolls).
Names were often etched into ossuaries (burial boxes). This one dates to first century Palestine.
For example, the two most common names in both the Gospels and in first century Palestine were Simon and Joseph. These two names make up 15.6% of Palestinian Jewish population and 18.2% of the Gospels. The nine most common names among Palestinian Jews was 41.5% and 40.3% in the Gospels. These are near exact matches.
This is especially striking when you compare them to the Gnostic Gospels, which include several exogenous Egyptian names, which indicates the setting in which it was written. Again, frequency of common names is not something easily replicable or even discoverable without modern archeology. This is strong evidence of historical authenticity.
(3) Lastly, there are all these wonderful, unplanned, small signs of historical accuracy in the Gospels. For example, there is a correspondence between Josephus' and the Gospel account of Herod Antipas. From Josephus, we know that Herod Antipas went to war against his neighbor, King Aretas of Nabatea in AD 36. Herod had been married to Aretas' daughter, and after a long marriage, divorced her to marry Herodias (the wife of his half-brother), thus provoking conflict with Aretas. According to Josephus, when Herod was defeated in battle, the people saw it as the justice of God and the vindication of John the Baptist. Josephus does not tell us exactly how and why John the Baptist was involved. But from the Gospels, we learn that John the Baptist had publicly preached against Herod's marriage to Herodias, and was arrested and ultimately executed for this. The two pieces fit together perfectly: each source, Josephus and the Gospels, providing parts of the story that complete each other.
Salome dancing before Herod by Rochegrosse
We could add to these many other pieces of evidence. The attestation of non-Christian writers (Pliny the Younger, Tacitus), the existence of hundreds of fragments of the Gospels dating to the 2nd and 3rd centuries, etc. Each one, by itself, can perhaps be explained away. But at a certain point, Occam’s razor comes into play.
Occam’s razor is the logical principle that the explanation that requires the least number of assumptions is usually the correct one. In other words, the simplest explanation is usually correct. In this case, Occam’s razor would state that in the first century, there was a rabbi named Jesus of Nazareth who did remarkable healing miracles, taught about the Kingdom of God, died on a Roman cross, and then rose from the dead as vindication of his identity, and that his earliest followers accurately recorded his life and teachings in the writings of the New Testament.