Religion is based on each independent person, and one should not attempt to enforce the rules of their religion onto another

Any time someone forbids murder, they're "enforcing religion on someone else" (Thou shalt not kill). Religion isn't based on individual preference. Their choice of religion may be, but religions themselves are based on truth or falsehood. Separation of church and state means you can't force someone to practice your faith, not that a society's laws can't be inspired by religious ethics. When you take that away, you're left with the state atheist governments we saw in the USSR and China.