The primary difference between the two was the method of governance. Under the Republic, the Roman Senate comprised primarily of members of the Patrician families, had full control over all foreign affairs and financial matters. Additionally, two Consuls were elected each year. They had to be at least 40 years old to serve. The most respected man in the Republic was the Censor who had the power to prosecute Senators for misconduct. Finally, a religious official known as the pontifex maximus who served for life was responsible for determining appropriate days for the conduct of business, setting the dates for movable feasts, and was also responsible for reading auspices, or oracles. The Roman constitution also provided for the appointment of a Dictator during times of crisis who had absolute authority for six months. The purpose of this arrangement was to preserve the stability of the Republic.
After the rise of the Empire, the Roman Senate remained, but was subservient to the Emperor, whose rule was virtually absolute. A codification of Roman law, first adopted during the Republic known as the Twelve Tables was developed into a rather elaborate legal system which became the basis for Western legal tradition. During the Empire, the army became a permanent fixture of the government; and the area under Roman control expanded substantially. Roman society changed very little if at all from the Republic to the Empire; the head of the family continued to be the pater familias who exercised extensive family control.
Among the greater changes, it was during the Empires tenure that Christianity became the official religion of the Empire. The religion was born during the reign of Augustus, and later became prominent. During the Republic, the old Greek Gods and a number of cult religions had been prominent.