I heard that one who has doubts about Allah S.W.T will burn in hell. Is that true ? Does yakeen or strong conviction means a person has no doubts about Allah S.W.T at all. Please make this clear. Please tell me does a person becomes kafir for having doubts about the existence of Allah.? Allah S.W.T says : The (true) believers are those only who believe in Allah and His messenger and afterward doubt not, but strive with their wealth and their lives for the cause of Allah. Such are the sincere.Quran (49:15). So does someone who doubt the existence of God becomes kafir ?
Contrary to the previous answer, which cites Wikipedia as the go-to place for knowledge about Islam, having doubts about Allah or Islam does not make one a kafir, whose meaning derives from "to cover". So a "kafir" would be, not one who doubts or uses his/her mind in considering the issue of faith and a supreme power, but rather one who seeks actively to cover the truth. Also this would apply to those who actively try to suppress the truth. One of the greatest sins in the Quran is oppression, and also repression, of truth. Also, the Quran frequently enjoins people to "use their minds" -- an expression in English that gives the right impression of the Arabic, except that the Quran refers to the mind not as a noun but only as a verb, to think, but I the Quran thinking is done in the "heart", which again as in English means the "core" of a person, his free will understanding.
It goes beyond the parochial idea some have about Islam, that its adherents worship the Prophet Mohammad, which would be considered idolatry and kufr ... because worship of any created being or thing is a form of denial of Allah. This is actually a profound concept but many refuse to use their minds to deepen their understanding of it. But the word worship should not be confused with honor or even sacred. Life is sacred, but not something to "worship" per se. So we honor Prophet Mohammad but do not worship him.
So what then is the case of the doubter? If the doubt occurs in the process of free thought, considering the idea of God/ Allah — and it is a very abstract yet real idea, which is important! — and as long as the idea is "in play" and not settled on and decided on and kept as your internal guiding dogma, then you are in a state of flux, one could say, but not kufr. Once however you settle on that doubt as your faith and your creed, that by which you guide your actions, then that would make you in a situation of "covering up" the truth, not seeking it, and deceiving yourself as having arrived at some conclusion.
To a disbeliever, denial of Allah's existence is an article of faith. But it seems strange to those who base their logic on no God and also (comes with the territory) an essential meaninglessness to existence or randomness, that one would think of God as truth and denial of God as deceit. Muslims on the other hand think it strange that one would see the heavens and the earth and other signs in mathematics and in their own consciousness, and then base their reality on "nothing matters" and nothing has any intrinsic meaning or value, and there is no unifying principle, but further, no essential goodness, no Creator, no supreme Power to whom we all return. The nature of the Islamic faith unlike Christianity as it is mostly practiced today is that Islam does not oppose logic. There is no logical argument against he existence of God, who is also beyond proof, insofar as proof is a type of "control" and Allah cannot be, as supreme power, controlled because God alone is in complete control.
Just note that the depth of this reality, the existence and dynamic always living presence of God/ Allah is beyond what you can "wrap your head around." It's a decision. Deciding to consider Allah as Almighty, All-Merciful and to be worshipped then opens up a door, an entrance (fatiha) into an understanding of how our lives are deeply meaningful and what we do matters more than we can imagine, and there are consequences after this life. Considering this possibility as true for a moment, feeling the weight of it and its depth, that is the flip side of doubt. You need to consider both sides to determine if you will surrender your ego to Allah or live for this life and for yourself, guided by your ego and desires, enjoying the temporary feeling that comes with thinking that nothing really matters. But that too has consequences.
All the warnings in the Quran are ultimately in your own interest and not a sort of in-club membership denial. It is good to take such matters seriously as I believe you do.
Thoughts do not count, but rather your speech and actions. Other than for those who have achieved yakeen (closeness to Allah) and ihlas (purity of faith) it is normal to have doubting thoughts.
This is the test of sheytan, these waswasi thought whispers and doubts. You must pass these tests in order to achieve yakeen and ihlas. While the blessed few achieve yakeen and ihlas in relatively short time, for many Muslims this takes years and for some a lifetime.
So don't worry doubting thoughts don't make you kufur. It's just sheytan at work. If you are having the same doubting thoughts, it is a sign from Allah that you need help and advise from alim (leaned people of Islam). After more than 1400 years of history, there are no doubts that haven't been already addressed. If you can point out what your specific doubts are as a separate question we can surely help you InshAllah.
Now if you made a conscious decision to reject Allah and his messenger and openly declare this in writing or orally in front of witnesses. Then you will be kufur.