Don't use google maps for subway directions in Boston.
Use http://www.mbta.com
www.mbta.com estimates 51 minutes using the silver/red line options already suggested. I used a 2:30pm departure from Terminal B logan airport on Saturday may 21st. (Day makes a difference, the T runs a different schedule on weekends than it does on weekdays). Heres is what I got:
Silver Line South Station
2:42 PM Depart from Terminal B Stop 1
3:03 PM Arrive at So Station Silver Line (inbound)
Take Red Line - Alewife To Central Sq - Outbound
Approx. 3:13 PM Depart from South Station - Inbound
Approx. 3:22 PM Arrive at Central Sq - Outbound
Walk For 11 Mins. To 134 Norfolk St Cambridge Ma
Walk a short distance SE on Massachusetts Ave.
Turn left on Essex St.
Walk approx. 2 blocks NE on Essex St.
Turn right on Harvard St.
Walk approx. 1 block SE on Harvard St.
Turn right on Norfolk St.
Walk a short distance SW on Norfolk St.
Another option might be St Anthony's Shrine in downtown Boston. It would be less than a 10 minute walk from South Station and they have several masses on Sat afternoon and Sunday morning.
http://www.stanthonyshrine.org/mass.asp
And there's other Boston options ...
Is there a reason why you want to attend Mass in Cambridge?