Ever since their emergence as a major art form in the Hellenistic era, mosaics have been prized for the glittering radiance of their colors, their permanent, almost eternal nature, and the painstaking craftsmanship required to create them. In late antique and medieval Italy, mosaics were the most important medium for monumental religious art, just as frescoes would be in the Renaissance. In fact, the mosaics that adorn the fourth to sixth-century churches, baptisteries, and mausoleums of Rome, Ravenna, Naples, and Milan are among the first examples of Christian pictorial art on a grand scale.