Revealing My Girlfriend's Beliefs to Her Mom: Did I Overstep?

My girlfriend grew up in a deeply religious family, but around two years ago, she began having doubts. It's not that she despises the religion; she just felt it wasn’t right for her. We met a year and a half ago, and have been together for a year now. When we first met, she was still unsure, but about five months ago, she confirmed she was agnostic. She hadn’t told her parents because she wanted to be absolutely certain.

Earlier this week, we were out and bumped into her mom. She approached us and started a conversation, asking who I was since we hadn’t met before. My girlfriend hesitated, so I stepped in and introduced myself as her boyfriend and mentioned that she was no longer following their religion. Both my girlfriend and her mom were taken aback. My girlfriend asked why I revealed that information. I told her that since she had decided long ago that she wasn’t Muslim anymore and we’d been together for so long, there was no need to keep it a secret any longer.

My girlfriend and her mom walked away to talk. Her mom was shocked but said she’d come to terms with it and was just glad that my girlfriend was happy. I thought it all went well.

However, later my girlfriend messaged me saying I overstepped and it wasn’t my place to share that information. She planned to tell her parents in private when she felt ready. I argued that things turned out okay. She called me an asshole and said I betrayed her trust. Her siblings and friends, who already knew, also messaged me, accusing me of not knowing when to keep quiet.

So, even though things seemed to turn out fine... Did I mess up?

I sometimes wonder how people would react if this happened on a reality show. Would they see me as the villain for speaking up or think it was justifiable given the situation? The drama would definitely be intense.

Did I make a mistake by telling my girlfriend's mom about her leaving the religion?