The religion became "popular" because if you refused to join it, you were executed.
Christianity was the religion of all western Europe by the year 1100.
Christianity I believe
religion was the strongest influence in western Europe
Medieval Monks spread Christianity by many different techniques. Now, if you are talking about Medieval Monks in Europe, then they spread Christianity by telling others mostly. There is a website all about Medieval Monks below.
Christianity is predominant in the western regions of the world, such as America and Europe.
Western Europe does not have a central religion. But, the largest religion in that region would be Christianity.
The areas of Europe that likely believed in Christianity was the western part of Europe, which is Spain and Italy.
Europe and the Western Hemisphere