Rome is the city with the most churches in the world. There are about 900 of them.
Many of them can be visited for free. First of all the four Papal Basilicas, namely: St. Peter's Basilica, the largest church in the world (the only one that cannot be visited for free).
The Basilica of Santa Maria Maggiore, an absolute masterpiece of art and architecture. The Basilica of St. Paul Outside the Walls, where there are representations of all the Popes, from St. Peter to Pope Francis. The Basilica of San Giovanni in Laterano, defined as the "mother of all churches".
Also worth visiting is the Church of San Luigi dei Francesi, located between the Pantheon and Piazza Navona. From an artistic point of view, the church is an exaltation of France through the representation of its saints and its greatest historical figures starting from the facade which houses the statues of Charlemagne, St. Louis, St. Clotilde and St. John of Valois.
Not to be missed is the famous Contarelli Chapel where you can visit the works of Caravaggio for free. In fact, inside is the famous triptych composed of The Martyrdom of St. Matthew, The Calling of St. Matthew and St. Matthew and the angel.