The hundreds of churches of Rome
We do not know exactly how many churches there are in Rome, but there are hundreds of them, many of them worthwhile to visit.
The St. Peter's is considered to be by far the largest and most famous of the Churches of Rome. Many do not realise that this church is not actually in Rome, but in Vatican City! The world famous Pietà by Michelangelo and Bernini's Square alone are more then enough reason to visit this church.
Visit also the other Three Arch basilics of Rome! The San Giovanni in Laterano, Santa Maria Maggiore and San Paolo fuori le Mura. These beautiful churches all have a distinguished papal altar, a papal seat and a Holy Door, which is only opened by the pope and only in a Holy Year.
But the many smaller churches in Rome are also real treasures. Visit with your guide churches like the San Clemente or the San Luigi dei Francesi and you won't be disappointed
Some useful links:
San Clemente Basilic; http://www.basilicasanclemente.com/
Santa Maria Maggiore; http://www.vatican.va/various/basiliche/sm_maggiore/vr_tour/index-en.html
San Giovanni in Laterano; http://www.vatican.va/various/basiliche/san_giovanni/vr_tour/index-en.html
San Paolo fuori le Mura; http://www.vatican.va/various/basiliche/san_paolo/vr_tour/index-en.html