Where was Christianity most strongly established by A.D 325 and why do you think so

By A.D. 325, Christianity was most strongly established in the Roman Empire. The primary reason for this is the conversion of Emperor Constantine to Christianity and the subsequent Edict of Milan in 313, which granted religious tolerance to all religions, including Christianity. 

Constantine's conversion played a crucial role in the acceptance and spread of Christianity. With the Edict of Milan, persecution of Christians ceased, allowing the faith to flourish. Additionally, the First Council of Nicaea in 325, convened by Constantine, aimed to address theological disputes within Christianity and establish a common doctrine. This helped in providing a more unified and standardized version of Christianity.

The patronage and support from the Roman Empire, combined with the end of persecution, contributed significantly to the rapid growth and establishment of Christianity as the dominant religion in the region.