I have a next to exactly similar situation as you Kevin. I was also baptized as a Roman Catholic, and I was forced to sit through Wednesday- Jesus- school. I've debated the topic of religion with my mom many a times, and it usually gets ended by my mom getting overly emotional, yelling at me, etc.
But, I also am in the Boy Scouts, and one of the requirements to becoming an Eagle scout is to have faith in a God. Whether it be the Christian God, or Buddha, or even Ra the Sun God. Point is, we're supposed to have some sort of faith in something that has greater power and control than us on Earth. I completely disagree with organized religion and their corruption, the stories of miracles that happened thousands of years ago, and prodigies today that claim to be prophets of God.
What I decided, is that I am more or less agnostic. I am not positive there is no God, and I am also not positive there is a man in the clouds watching over us. I decided to believe in a higher power that can create such beauty as the Earth around us, and the infinite universe surrounding us. The strong emotions of love, happiness, and the connections we have with people are also almost magical in a way. I'm aware chemicals in our brain have a major part in it, but regardless, it doesn't hurt to at least think there is something greater out there.
Overall, my reply to your question would be this. Don't lie. If your mom can't accept your thoughts and opinions, then it isn't worth trying to talk to her about the subject. Saying, "I give up, I believe in God" is weak sauce. Be honest. But also don't make absolutes. If there is a remote chance of you believing in a God of some sort, then tell your mom that. Don't tell her you absolutely are not in agreement with the thought of God. Don't say you completely don't believe in anything, unless that is the case.
And also remember, you're a teenager, and depending on how your life goes, and your experiences, your views may change.