Montessori itself is not religious. It is, however, not trademarked so anyone could call themselves a Montessori school. There's nothing to stop a Catholic parish, or any other religion, from setting up a religious Montessori school.
How religious the school is depends on a lot of things. It may not be very religious at all: many Catholic schools cater to non-Catholic families. I went to a Jewish preschool, despite not being Jewish. It was just very close to my house and a very nice little school.
Trying to live a simple life in a messy house in a complicated world with : DH, DD (b. 07/07), DS (b. 02/09), and DD (b. 10/10)