Is the mujtahid correct regarding matters of the greater fiqh (usul al-din) in which ijtihad is permissible, or in matters that will only be known on the Day of Judgment, such as the issue of the disbelief of one who abandons prayer out of negligence, or the nullification of deeds due to hypocrisy, or whether a specific act is a major or minor sin?

There is no difference between fundamental and subsidiary issues in that they are subject to ijtihad, and the mujtahid (one who exercises independent reasoning) is rewarded even if he errs. Every mujtahid whose tools are complete and who meets the conditions of ijtihad is correct if he exerts his utmost effort. This does not mean he is correct in discerning Allah’s ruling in the matter itself, but rather, the one who is correct in discerning Allah’s ruling is one, and he is rewarded twofold, while the one who errs is rewarded once. The aforementioned issues are among the matters of ijtihad that are subject to scholarly disagreement among jurists. Thus, the scholar who exercises ijtihad in them is rewarded even if he errs. As for the layperson, he must follow someone he trusts among the people of knowledge. And whoever believes in a permissible opinion that has been stated by one of the accredited scholars, there is no denunciation or punishment upon him. We warn against indulging in waswas (whispers/obsessive thoughts).