The ancient Egyptian religion, with Isis, Osirus, Horus (another "holy family") went on for around 3000 years. I'm sure someone 2000 years into that would have said it was eternal, it had never stopped being practiced, it was believed by everyone that it was "Truth" and would go on forever. Today, we look upon it, and think it's silly to believe in such things, and absurd to have mummies and richly-filled tombs.
Note too, that as you point out early Christians were persecuted, but when Christianity took over the government of the Roman Empire (Constantine), then later when it broke up into many independent kingdoms - most being officially Catholic, and any belief but Christianity was outlawed, and believers in competing religion tortured and executed, all of society lost its scientific knowledge, mathematical knowledge. Instead of looking to science to explain or solve the problem with plagues, it was based on devils and witches, and a new frenzy of finding innocent people to torture and kill started. Galleleo was excommunicated and deemed a heretic for his astrononomical discoveries - he was not "rehabilitated" by the Catholic Church until just a few years ago.
History repeats itself if we do not learn from it.
