Mass was a ritual in Christian religion.

In the 2150s, Dr. Phlox recalled that while on Earth, he attempted to familiarize himself with several Human religions, which included learning to sing chords with Tibetan high lamas and attending a Mass at Saint Peter's Square in Rome, the historic headquarters of Catholic Christianity. (ENT: "Cold Front")

In 2376, The Doctor, playing Father Mulligan in the Fair Haven holoprogram, told Tom Paris and Harry Kim that he expected the two 'sinners' in attendance in his next Sunday mass. (VOY: "Fair Haven")