There is a shade of meaning between “I don’t believe” and “I don’t know how a person/I could determine that they/I affirmatively believe.”
I personally would interpret the former as non religious and the latter as agnostic, but it probably differs from person to person. Especially because non religious is often used to describe people who do not practice a religion, but may well still believe in it (though that would be non practicing for me).
There is a shade of meaning between “I don’t believe” and “I don’t know how a person/I could determine that they/I affirmatively believe.”
I personally would interpret the former as non religious and the latter as agnostic, but it probably differs from person to person. Especially because non religious is often used to describe people who do not practice a religion, but may well still believe in it (though that would be non practicing for me).