Is it a sin to take part in a non-Christian worship service?

I have attended multiple Jehovah’s Witness meetings, even saying an Our Father and singing some of their songs (though omitting stuff I could not agree to) at them. Is that a mortal sin, or does it fall under the First Commandment?

**Recommended Reading:
*]Are non-denominational Bible studies ok for Catholics?
*]Is it okay to attend a Protestant Bible study?
*]Why can’t we evangelize at a Protestant Bible study?
*]Is it a sin to worship in other Christian churches?
