This is an easy 2 hour walk around Rome to see some of the more fascinating religious sites.  

The Church of Santa Maria della Vittoria:  This is a good starting point for a walk around Rome.  A 7-minute walk from Roma Termini, not only is it a religiously significant church, it is an excellent example of the Baroque style of architecture.   Even with all of that said, inside is one of Bernini's great masterpieces, the mesmerizing Ecstasy of St. Teresa.  

Basillica Santa Maria Maggiore:  A short distance away and closer to Roma Termini is the church of the Popes. The church dates back to 432 AD.  It is the burial place of Bernini and Pope Sixtus V.  Under the altar is a statue of Pope Pius IX.  In a glass case are fragments of wood from Jesus’ manger.

San Giovanni in Laterano:  From Santa Maria Maggiore, walk straight down Via Merulan for about 15 minutes.  Otherwise, take the #714 bus right there.   Another way is to take the metro to San Giovanni, go through the old Roman wall and it will be on your left.

The original church opened in about 318 AD this was the first church Catholics could openly worship at in Rome.  It is the home church of the Pope and all Popes were “crowned” here until 1870.\

Santa Scala (Holy Stairs):  Directly across the street from San Giovanni in Laterano are the Holy Stairs.  They are said to be the stairs Jesus climbed on the day he was sentenced to death.  The marble stairs are covered by wood with glass holes  showing the stains of Jesus blood.  Each day, hundreds of people climb the stairs  on their knees.  At the top of the stairs, peek into the private chapel of the popes in the Middle Ages.  It is closed during lunch hours.

Saint Peter in Chains (San Pietro in Vincoli):  A 20-minute walk from from Laterano (or take the metro to Cavour or the #117 bus to the Colosseum).  The basilica was first built in the middle of the 5th century to house the relic of the chains that bound Saint Peter while imprisoned in Jerusalem. 

According to legend, when the pope held them next to the chains from of Peter's first imprisonment in the Mamertine Prison in Rome, the two chains miraculously fused together. 
The basilica also houses one of Michelangelo’s most powerful works, Moses, sculpted in 1515.