These strengths included a strong foundation, having been built off of the Roman Republic; the standardization across the empire of many aspects of life, such as language, law, and especially the extension of citizenship, which made the empire more cohesive and easier to rule; and strong leaders, who were able to utilize the manipulation of the upper class and Senate, and the management of the military.
It is hard to say without looking at your source (textbook or article if you have one associated with this question) but in the most recent studies, it has been Islam.
