Christianity was one of the most popular and well-organized religions in the late Roman Empire. It became tolerated due to its influential followers, though Christians were sometimes persecuted. Churches were built in the early Byzantine and medieval periods, featuring architectural elements like basilicas, apses, naves, and transepts. Byzantine art moved away from realism towards abstract religious symbolism in mosaics and icons. The Middle Ages saw the rise of kingdoms after the Western Roman Empire fell and the development of Christian scripture and Carolingian art under Emperor Charlemagne.