If we were to define humility without using word God or anything related to Faith, it has to be possible in the first place as that is when it truly can speak to all. Here is one. Humility is the acknowledgment of Truth in any given situation. That's it.
Mark of a spiritual person is to seek and identify the truth in any given situation. If we are inclined to a thought or a decision, one need to ask themselves, why am I thinking this way? That kind of search can be frightening because it will expose facades we have on us but that is required of a spiritual person. Are you ready?