Your parish has the option of enabling an RSVP feature when they post a new event.  If they have enabled this option for an event, you will see a small calendar-RSVP button on the top of the event.

Tap that icon and complete the small form that appears.

After you RSVP,  the icon will now appear as a calendar with a check mark to show that you have already  RSVP’d to the event.