There "may" be religious services on board but there is no guarantee. A few years ago Celebrity did have a priest or clergy program where a priest or minister would get their cabin for free but was required to perform a daily service. That program has been discontinued.
However, they do provide religious services for Catholic, Protestant and Jewish faiths during major religious holidays.
They also have a daily Catholic Mass onboard during their Holy Land Cruises
On other voyages a priest or minister, who is on board, can request time and venue for services. Or Guest Services will provide the name and location of churches in the port area.
__________________ Cruisemates Community Leader/Moderator
"There is a great difference between being well traveled and just having been to many places." ~Me