[Is it decisively judged that one who says “major sinners will exit the Fire” is a disbeliever or an immoral sinner?]

Answer (Allah is the One who grants success): Error in some foundational matters—even decisive ones—does not, by itself, invalidate faith. I will clarify a rule by which issues can be distinguished from one another:
We say: an error that does not entail ignorance of Allah—Exalted is He—nor attributing an ugly/evil act to Him: its doer is not declared a disbeliever, nor an immoral sinner, even if he contradicts decisive evidence—such as one who marries a woman in her ʿiddah (waiting period)—due to the generality of His—Exalted is He—saying: “And there is no blame upon you for that in which you have erred.” [Al-Aḥzāb:5]. And due to His—Exalted is He—teaching His servants to say: “Our Lord, do not take us to task if we forget or make a mistake.” [Al-Baqarah:286]. And due to the ḥadīth: “My Ummah has been excused for mistakes and forgetfulness…”
So their statement “we do not decisively judge him a disbeliever or immoral sinner” is built on the assumption that they erred—because they did not reach the evidences, or did not understand their meanings. But if they have accessed the evidences and understood their meanings, and know what is sound and unsound, then there is no excuse and no argument for them.
This is what that rule is built upon, and the statement of some of the Imams is built upon it as well. If stubborn defiance and obstinate rejection becomes apparent, then a decisive ruling of disbelief or immorality follows.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.3