Saint Paul's Church in Cambridge is very beautiful, I have never heard anyone use Cathedral in the same sentence. If you are looking for a Catherdral, Boston has one on Washington Street.
Saint Paul is located on Mt. Auburn Street at the end where it meets Mass Ave . Bow and Arrow Streets run behind it. It should be in the 27-30 number range of Mt. Auburn Street.
It is easy enough to get by taxi and should be in the range of $60.00.
You can take the sliverline from the terminal to south station and change to the red to Harvard Square.
www.mbta.com
cab should take 25 minutes, the t between 45 minutes and one hour.