Will whoever has an atom's weight of faith in his heart not enter Hellfire, and whoever has an atom's weight of arrogance in his heart not enter Paradise?

No one who has an atom's weight of faith in their heart will remain eternally in Hellfire. The Quran and Sunnah (definition) both agree that if a disobedient Muslim dies without repenting, their affair is with Allah. If He wills, He may forgive them, and if He wills, He may punish them, but they will not abide eternally in Hellfire. It has been narrated in a Hadith (definition) that whoever says "La ilaha illa Allah" (There is no god but Allah) and has in their heart the weight of a barley-corn, a grain of wheat, or an atom's weight of good, will be brought out of Hellfire. As for the statement, "No one who has an atom's weight of faith in their heart will enter Hellfire," it is interpreted to mean that they will not enter it like the disbelievers (i.e., they will not abide eternally in it), for some disobedient believers may enter it and then be brought out through intercession.

As for "No one who has an atom's weight of arrogance in their heart will enter Paradise," its meaning is that they will not enter Paradise without facing the possibility of retribution and punishment. They may be punished or forgiven, unlike the obedient believer who is free from arrogance, who will enter it without such a possibility. In summary, it is not impossible for someone who has an atom's weight of faith in their heart to enter Hellfire, no matter how many major sins they commit. And it is not impossible for a Muslim who has an atom's weight of arrogance in their heart to enter Paradise, no matter how much faith they possess. The Khawarij and Murji'ah went astray in understanding these texts, while Allah guided the Ahlus-Sunnah to the true position that reconciles the texts.