Final answer:
The dominant religion in Europe in 1500 was Roman Catholicism, under the influence of the Vatican in Rome, before the Protestant Reformation began.
Step-by-step explanation:
According to the inset map, the religion that dominated Europe in 1500 was Roman Catholicism. During the 13th century, conflicts between followers of Jesus and Muhammed continued, with Christian forces retaking the Iberian Peninsula and pushing Islamic rule out of Spain and Portugal. The Vatican in Rome held great spiritual and political power across Christian Europe, making the Roman Catholic Church the unifying religious structure of the continent before the onset of the Protestant Reformation in the 16th century.
By the beginning of the 16th century, the Protestant Reformation had not yet started, and challenges to the Roman Catholic Church were minimal. Largely, the entire population of western and central Europe shared a common religion, Roman Catholicism, with the Catholic Church being the dominant force in spiritual and secular matters. This unity held until Martin Luther's reform efforts in 1517, which initiated a gradual but profound shift in the religious landscape of Europe.