What's up with churches and guitars in this joke from The Big Bang Theory?

From The Big Bang Theory S10E12 : > Mary: Thank you, God, for the food we are about to receive and for the nourishment of our bodies and bless the hands that prepared it. Amen. > > Sheldon: Given that your hands prepared it, isn't that a little self-serving? > > Mary: You start changing the words to the prayers, next thing you know, you're in a church with a guitar. --- Not sure if this is to do with Christianity or some TBBT inside joke I'm missing like Sheldon changes words to prayers at home and then later changes words to prayers in church.