During the early years of Christianity, the religion was considered illegal. Despite this, Christianity was able to spread quickly. People who practiced Christianity had to hide from authorities, and the religion spread secretly, while Christians faced persecution, until Constant declared it the official state religion of the Roman Empire. After this point, Christianity became even more popular.
It is very likely that, if Christianity had not been declared legal, the speed in which it spread would have been a lot slower. However, it would have continued to spread through secret methods in the same way that it had during its early years.