You'll probably see Cathedral of the Holy Cross from a distance before you actually find this important religious building. Rest a while inside if it's open or just take in the facade from the street.
If you're spending a while in South End, Cathedral of the Holy Cross is a fine place to kick off your exploring. This principal church is one of many attractions worth seeing here. Fenway Park, Boston Common, and Faneuil Hall Marketplace are also waiting for you close by.