Rome did not like that Christians were taking the power away from the Roman government so they began persecuting them. Christians, in spite of the persecutions, were able to continue to grow and share the gospel.
Constantine reunited Rome and claimed to be the first Christian ruler but he did not understand the true meaning of Christianity. He used violence and force in the name of Christ and demanded that everyone follow the same religion.
Throughout the years, Rome became divided and began to weaken. The neighboring countries began to slowly take over Rome, bit by bit. Rome fell due to its foundational roots of violence and force.