For having such a pagan history despite it's "Christian claims", the over abundance of worship centers in Rome is astounding. Exactly how much "worshipping" takes place these days can be your guess, but it's no waste for the structures which further enhance and overload the arhitectural and museum themes with their artistic wonders both exterior and interior. Going fuzzy in the mind from looking at too many piles of ruins is one things, but when the same symptoms take place with the overwhelming, magnitudal amounts of arts one's seen, I was somewhat troubled with my purpose of a tourist and why bother coming? I'd about seen all I could intake, still feeling compelled to look. Yet I no doubt short-changed the opportunities of this trip because I couldn't handle processing any more.
Santa Maria Maggiore in the Esquiline District and Santa Maria in Trastevere Cathedrals were by far the most decorated and prime examples of what my words likely inadequately relayed in the first paragraph. We're talking every inch of space covered with art from the gold-gilted and frescoed ceilings to the mosaic tiled frescoed floors! The more you know about Catholicism definitely helps to appreciate all you'll see in the side chapels, and smaller cupolas are often more detailed than the grand domes.
Other churches not to be missed are San Pietro in Vincoli (near Parco Opello) which house Michaelangelo's Moses statue and the chains which supposedly bound St. Peter. Santa Maria della Vittoria (southern rim of Via Veneto district) was built to resemble a theatre with statues watching angels ravish St. Teresa in Bernini's "ectasy" creation. Nearby Palazzo Barberini is also worth passing through.
One of the older, more interesting churches lies atop the Aracoeli Staircase...about double the amount of steps it takes to get to my 6th-floor walkup apartment. Stop half-way up and catch your breath while enjoying views around the city and back to the side for the Capitoline Hill before continuing to enter Santa Maria in Aracoeli; another artistic jackpot definitely worth your efforts.
You'll undoubtedly find many hidden and favorite treasures of your own. Also, no church is too small to NOT be worth your time to duck into for a quick browse. For all, it's handy to have Lira coins to drop into small light machines which further illuminate the works of art you'll be viewing. Persistence will also pay off for "lesser churches" which aren't as valid on open hours as posted in front or listed in guidebooks.