My husband and I attended Mass while staying at Sans Souci. We went with another couple and split the cost of the taxi. It was a lovely church (Our Lady of Fatima) right on the water and we were made to feel most welcome. It was about a 7-10 min. ride.
CTI usually organise a bus to the local catholic church for mass on Christmas morning. It's a fabulous church, open windows and overlooking the caribbean we loved the service and the children performed a short nativity one year.
Although the hotel is usually almost fully booked over Christmas and New Year it's never crowded and there's always room at pool, beach, etc. don't know how they do it - it's a magical place!
We'll be there Christmas and New Year again this year and just can't wait to get back home as we had to spend last Christmas at CSS, which is great but it's not CTI.