Atheist Christians?

In his book, The Prodigal God, Tim Keller makes the claim that the early Christians were accused by the Roman Empire as being atheistic. Tim Keller didn’t provide a citation.
Then today I was watching an episode from Christianity: The First Two Thousand Years and John Dominic Crossan makes the exact same claim except that he adds that the Roman Empire saw the Christians as a hateful sect because of their separatist lifestyle in that they refused to participate in the pagan festivals. I am just wondering from what historical source do they get all of this. Shoot me a link of anyone knows.