For most of my life, I was a nondenominational Christian, meaning I believed in Jesus Christ and Heavenly Father, but I never did anything about it.
I'm going to go back and say that my original interpretation of this sentence was correct.
Mary Cate has a Relief Society activity tonight, which means that I need to figure out what I'm doing for dinner.
I'm not defining anything, but I'm describing a ramification of her having the activity tonight.
Or, to use the author's slightly odd construction,
Mary Cate has a Relief Society activity tonight, meaning I need to figure out what I'm doing for dinner.