"If you don't believe in God then why do you say 'Oh my God'?"
Religion doesn’t own language and it doesn’t own words.
Back in ye olde days people chose to insert religion into colloquial language. Now you have to live with the fact that non-religious people use religious language because the colloquial language contains religious terms.
When Muslims say “What did I do wrong in a past life?” are they confessing to believing in reincarnation? No.
Are Jewish people who say “Karma’s a bitch, huh?” admitting they’re actually Hindu or Buddhist? No.
Are Christians who use the term "soulmate" actually believers in the Greco-Roman gods since the concept comes from an Ancient Greek myth? No.
Are atheists who say “Oh my God!” admitting to believing in a god? No.