If “I am a believer, God willing” means “I may or may not presently believe,” the speaker expresses doubt about existing assent; this conflicts with the decisiveness required for faith.
By contrast, a person may be certain of present belief yet not claim knowledge of whether they will remain faithful at death. In that case, uncertainty concerns future perseverance, not present conviction.
The distinction is logical as well as theological: not knowing tomorrow's condition does not imply ignorance about one's current assent. Confusing the two produces a false disagreement about all forms of qualification.