How did christianity flourished withih the roman empire?

despite persecution there where bible students who pushed to teach the bible and preach even facing death. the apostle paul was locked up and beaten with rods for preaching. Even later in history you havd men like Martin Luther who fought the oppresive catholic church, and said teach from the bible not personal doctrine. he wrote the 95 thesis and because he didnt repent he was exiled. even there im sure he preached.