How bad was this faux pas in synagogue?

My family is Jewish and when growing up, we attended synagogue on high holidays and occasionally at other times. We seemed to bounce between orthodox a (conservative - thank you, u/Pikarinu) and reform synagogues, as we were somewhere in between.

As a kid, prior to the service one time, my dad was recruited to carry something that required a few men to lift. I believe it may have been the ark, but I'm not sure - it obviously had some significance. It was my dad and the rabbi and at least one other guy.

My dad was struggling under the load and grumbled , "Jesus Christ..." That was a kind of habit of his - he'd say that on occasion, sort of under his breath, but certainly loud enough for others to hear.

My dad was a doctor and I didn't often see him reprimanded, but the rabbi sure gave him a stern look in that moment.

So how bad was this?