How and why did Christianity spread throughout the Roman Empire?

Christianity, sprang up in the corner of the Roman Empire. At first, Christianity was just one of many religions practiced in the empire. but despite many obstacles, the new faith grew rapidly, and by A.D. 395, it had been declared the official religion of the tRoman Empire. It gained strength,when the roman empire fell, the christian church took over much of this role, becoming the central institution of western civilization for nearly 1000 years