1. If all religions are true, then Christianity is true and valid, yet an essential claim of Christianity is that all other religions are false, taken as a whole.
2. Either Christianity is true and others are false, or others are true and Christianity is false.
3. Either way, all religions can’t be true.
Contradiction: All religions are true. All religions are not true.
So, the next time someone claims that all religions are equally true and/or valid, you’ll be ready to demonstrate how this view defeats itself!