Bart D. Ehrman, The New Testament: A Historical Introduction to the Early Christian Writings, sixth edition (OUP, 2016), 85-86.
Suppose I am a Greek-speaking worshiper of the goddess Artemis from Ephesus. I listen to a stranger passing through town who tells of the wonders of Jesus, of his miracles and supernatural wisdom. I become intrigued. When I hear that this wandering stranger has performed miracles in Jesus’ name – my neighbor’s son was ill, but two days after the stranger prayed over him, he became well – I decide to inquire further. He tells of how Jesus performed great miracles and of how, even though he was wrongly accused by the Romans for sedition and crucified, he was raised by God from the dead. Based on everything I’ve heard, I decide to forego my devotion to Artemis. I put my faith in Jesus, get baptized, and join the local community.
I later take a trip for business to nearby Smyrna. While there, I tell friends about my new faith and the stories I’ve learned about my new Lord. Three of them join me in becoming Christian. They begin to discuss these things with their neighbors and friends. Mostly their beliefs are rejected, but they acquire several converts, enough to come together once a week for worship, to discuss their faith, and to tell more stories. These new converts tell their own families the stories, converting some of them, who then take the word yet farther afield.
And so it goes. As the new converts tell the stories, the religion grows, and most of the people telling the stories are not eyewitnesses. Indeed they have never laid eyes on an eyewitness or anyone else who has.