What was the roman empire's views on christianity at various points in its history? what caused them to change over time?

Prior to 380 AD, the Roman Empire religious views consisted of the worshipping of many gods, or polytheism. Even though they worshipped many gods at this point, Romans were deeply religious and made it a point to live in a way to please the gods. There was no separation of church and state and priests were often also high ranking officials-sometimes even the Emperor.

After a period of instability, the Republic collapsed and a new Roman emperor (Augustus) was put into place. With this new empire, "emperor worship" became a focal point. Anyone who rejected this "state religion" was seen as being treasonous against the empire.

Monotheistic , the worship of one god, Judaism started to spread as a "true religion". The Seige of Jerusalem led to the fall of Jewish political power. Christianity emerged in Roman Judea around 1 AD and continued to spread throughout the Roman Empire.  In the early 4th Century, Constantine I became the first Christian emperor.

In 380 AD, the Edict of Thessalonica was passed by Emperor Theodosius marking Nicene Christianity as the main religion of the Roman Empire.