What is the ruling on the saying, "People know Allaah through reason"? Does it mean that there are people who do not know of the existence of Allaah? If so, why does Allaah the Almighty punish them after death even though they are ignorant of His existence?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

Aspiring to know the Creator, the Almighty, is within the innate disposition of humans. A sensible person who contemplates himself and the universe around him will realize that there is a Creator Who is perfect beyond deficiency and blemish.

Contemplation is one of the ways to know Allaah the Almighty, to which the Quran draws our attention. Allaah the Almighty Says (what means): {We will show them Our signs in the horizons and within themselves until it becomes clear to them that it is the truth. But is it not sufficient concerning your Lord that He is, over all things, a Witness?} [Quran 41:53]

The universe, as said by a scholar, is an open book that is read by both scholars and ordinary people.

This, however, does not mean that reason on its own and apart from revelation is sufficient. Rather, reason lights a person’s way towards knowledge and helps him to understand revelation. Therefore, Islam considers the mind the first factor of eligibility for receiving religious assignment. It was narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said that three kinds of people are not burdened by religious obligations: the one who is asleep until he awakes, the child until he reaches puberty and the insane person until he becomes sane. [Ahmad, Abu Daawood, At-Tirmithi and An-Nasaa'i]

The second way to know Allaah the Almighty is through the content of the texts of the Quran and Sunnah. You can read more details about this in Fatwa 81529.

Out of His Grace and favor, Allaah the Almighty does not burden His servants to obey Him merely based on their innate knowledge. Therefore, the people who were not from the prophets' period and their like, including those whom Da‘wah (the Islamic call) never reached, are not to be punished until they are educated beyond any excuse of ignorance and doubt. Allaah the Almighty Says (what means): {And whoever errs only errs against it. And no bearer of burdens will bear the burden of another. And never would We punish until We sent a messenger.} [Quran 17:15]

It was also narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said that such people will be tested on the Day of Resurrection. [Ahmad]

Allaah Knows best.