السلام عليكم ورحمة الله وبركاته، أحسن الله إليكم what does استثناء in iman mean and what does استثناء mean?

To say I Am Mu'min, a believer, in sha'a Allah. And this is not doubt as sheikh Al Islam Ibn Taymiyyah has shown in his book Al Iman Al Awsat, however it is only from their fear that they have not done the obligation upon the way they were commanded with. And Imam Ibn Baz said What is meant is not doubt, but rather the phrase “In shā’ Allāh” is used because a person does not know whether he has completed his Iman or not.