The decline of the Roman Empire was a period of enormous change in Western Europe. There are several reasons that contributed to it, and that in turn helped the rise of Christianity.
During the 3rd century AD, the Roman empire suffered a crisis due to foreign invasions and wars. The Sassanid Empire was a constant threat, while repeated civil wars and plagues weakened the empire from the inside. The Rhine frontier was contantly threatened by "barbarian" groups, which had grown in population and improved their technology. The size of the empire made it difficult to control and protect, and eventually the need for division became obvious. Each half of the empire was controlled by a different emperor, and mistrust and lack of communication caused other political conflicts. There was also a rise in corruption and violence within the government, which created or exacerbated social divisions.
It is common for religions to grow in times of trial because they provide people with an explanation for chaos. It also gives the population hope for a better future or a fairer society. Finally, religions often increase social unity and community ties at a time when those might be endangered.
