For the first 300 years, Christianity largely survived in the catacombs—Church the same way as Christ in birth and evading Herod’s intent of murder—underground.
The first popes were much as leaders of a POW camp—the most senior still surviving and in further parallel, when wine was tortured to the point of death, another was appointed.
Christianity survived in those days, in spite of world’s aim at eradication—genocide, though it crossed peoples and preached message to all the world.
Christianity’s message was radical love—the value and merit of every living soul in a world that thought nothing of such.
In like time from nascent through first centuries of practice—Islam spread by sword. It did not so much change and convert as it simply murdered or replaced. It had no such place for free will and God’s mysteries but simply coerced and demanded submission.
Oftentimes, the fundamentals of something are little changed from their beginnings.