How can one be certain that Islam is the true religion without merely relying on inheritance, especially since followers of other religions have the same certainty regarding their beliefs? What distinguishes this certainty from the certainty of others?

Many adherents of falsehood believe they are on the truth, as indicated by Quranic verses such as His (Allah's) saying: {Say, "Shall we inform you of the greatest losers in [their] deeds? [They are] those whose effort is lost in worldly life, while they think that they are doing good in work."} and His saying: {And whoever turns away from the remembrance of the Most Merciful - We will appoint for him a devil, who will be to him a companion. And indeed, they (devils) avert them from the [right] way, while they think that they are [rightly] guided.}

As As-Sa'di explained, whoever turns away from the remembrance of the Most Merciful (the Qur'an), a devil is appointed for him, who diverts him from the path and beautifies falsehood for him. There is no excuse for such people for turning away from the truth, even though they are able to be guided.

The Muslim and the non-Muslim are not equal in certainty and tranquility. The one who believes in the truth is not comparable to the one who believes in falsehood; each has their own state.

The truth is not known by people's affirmation, but by evidence and proofs. Whoever is in doubt must research and compare Islam with other [faiths] to discern the truth.

As for those who have not heard of Islam, they are among the Ahl al-Fatra (people of the interval) and have a specific ruling. Those who have heard of it but have not believed in it have various rulings depending on the establishment of proof against them.