Why was Christianity so popular in western Europe in medieval times?
Perhaps Christians were a bit harsh in the spreading of their own religion. They burned witches, but they also died and suffered for their own beliefs. They too were tortured and persecuted by the Romans. The Bible teaches that. But does it really matter why Christianity is so popular? You must choose your own path. You must choose for yourself what you think is right. I strayed from Christianity to pursue Wicca. A pagan religion. Pagans did sacrifice people a long time ago. So yes, there is something wrong with every religion. No one is perfect. Not Christians. Not wiccans. No one. We are all human.
The religion became "popular" because if you refused to join it, you were executed.
There were no results of the fall of Rome in Medieval Europe. The fall of Rome occurred in Late Antiquity. The result of the western part of the Roman Empire (the eastern part continued to exist for nearly 1,000 years) was Medieval Europe. The term Middle Ages refers to the period from the fall of the western part of the Roman Empire to the Renaissance (late 5th century-1500).