One was the dome on the temple to the Ancient Roman gods , called the Pantheon . This plan was in the form of an enormous Greek Cross with a dome inspired by that of the huge circular Roman temple , the Pantheon . The main difference between Bramante 's design and that of the Pantheon is that where the dome of the Pantheon is supported by a continuous wall , that of the new basilica was to be supported only on four large piers .