Saint Peter’s church in Rome was commissioned to have Arabic calligraphy

From the Dome of the Mosque of the Rock in Jerusalem to the great mosques of Isfahan in Persia, calligraphy decorated, enhanced and even helped to visually unify the greatest Muslim structures. The art of Arabic calligraphy was employed in many European churches as well, such as in Saint Peter’s in Rome. The representations of Christian saints that beautify the Capella Palatina in Palermo, Sicily, bear inscriptions in kufic, the early Arabic script. Today, the calligraphic tradition lives on throughout the Arab/Islamic world in religious, educational, governmental and commercial architecture.
Source: http://www.alhewar.org/ArabCivilization.htm