From archaeologist Rodolfo Amedeo Lanciani, The destruction of ancient Rome: a sketch of the history of the monuments, p. 36 f. (here):
To what use the temples were put immediately after the expulsion of their gods, we do not know; but it is certain that they were not occupied by Christians, nor turned into places of Christian worship. This change was only to take place two centuries later, when the scruples about the propriety of worshipping the true God in heathen temples had been overcome. In the year 600, Pope Boniface IV asked the Emperor Phocas for the temple which was called Pantheon, and turned it into a church of Mary the Virgin ever blessed.” Two periods, then, may be distinguished in the converting of pagan edifices into places of Christian worship, one anterior to the year 609, the other following that date. During the first, civil edifices alone were transformed, partially or completely, into churches; such were the Record Office, which became the church of SS. Cosmas and Damian, and the round market on the Caelian Hill, now S. Stefano Rotondo. After 609 almost every available building, whether secular or sacred, was made into a church or chapel, until the places of worship seemed to outnumber the houses.
This view, expressed by a 19th century archaeologist, is interesting. But is it true?