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?

These links should give you the answers that you need. If you have any further questions or concerns that are not answered by this, please contact Catholic Answers directly.
**Recommended Reading:
**
[LIST]
*]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?
[/LIST]

DISCLAIMER: The views and opinions expressed in these forums do not necessarily reflect those of Catholic Answers. For official apologetics resources please visit www.catholic.com.