The Basilica Spirito Santo (Basilica of the Holy Spirit) was originally consecrated by Arian bishops in the early 6th century AD and then converted to Catholic religion by archbishop Agnellus around 560. Today, the church does not significantly differ from its original appearance. However, the building is now 2 meters lower because in the 16th century the floor was elevated to counteract the phenomenon of subsidence. A 5th-century porch with five big arches on the front and one on the shorter northern side dominates the façade.
The basilica is divided into a nave and two side aisles, separated by fourteen columns topped by capitals and pulvinos. The 6th century Greek marble ambo is decorated on both sides with two crosses on a globe, while in the central curved part fluted small columns form three aedicules with shells and a tympanum.