Roman rulers opposed Christianity because the rulers believed that they were gods and wanted all of the people to see them as gods, which would not happen if the people believed in one god through Christianity. The lessening of the divine status of the Roman rulers would mean they would have less power in the lives of the people.
In 64 AD, the Roman Emperor Nero attempted to place blame on Christians when there was a fire in Rome. Nero did this to avoid blame. When Pliny the Younger came to power, he told his soldiers that Christians were not a threat and should not be harmed unless someone criticized them in a public forum and they would not let go of their beliefs.