Answer:
*This is personal preference*
Feel free to sub out words and change stuff, but this is just an example of what you could write.
I would write something along the lines of:
Rome became a political and economical powerhouse over the course of [x] years. Christianity/ money heavily influenced this empire's political climate as shown in/ demonstrated by [reference example from earlier paragraph]. From a historical standpoint, Rome was en route to becoming the most prosperous empire in the world, but too much focus on [insert] led to its downfall. Rome may not have been built in a day, but these examples have demonstrated that it could be burned in one. (last sentence is a reference to a fire, but could be elaborated on)
Step-by-step explanation:
So as you start to write more and more essays, sometimes you'll find that it's better to actually veer away from listing out topics all at once, and writing something that grabs attention (as an intro) and really drives home a point in the conclusion.
I think that summarizing a little bit of your intro is good (not the main thing you should do), but also hitting good keynotes through the content you wrote in the BODY is valuable and then wrapping everything up with something that is relevant but clever, such as a play on a common phrase or providing food for thought.
Hope this helps!