Anyone Can Call God “Father”

It depends on what you mean by “unbeliever.” If you mean an atheist, then it wouldn’t be an issue, as they don’t believe in God, let alone believe in him as Father.

On the other hand, if you mean someone who believes in God but is a non-Christian, the answer is a qualified “yes.” In a more limited sense than is normally used among Christians (see the Catechism 1265), one might argue that we are all children of God, insofar as we are all made in his image and likeness (Gen. 1:26-27) and because God desires all human persons to be saved (John 3:16-17; 1 Tim. 2:4; 2 Peter 3:9). In this sense, non-Christians might refer to themselves as children of God and thus, perhaps, as God being their Father-creator.