There is light in believers' faces, but disgrace in those of disbelievers

One of the secrets that God reveals in the Qur'an is that belief and disbelief are reflected on people's faces and skins. In many verses, God informs that there is light on believers' faces, whereas there is disgrace on disbelievers':

And you will see them brought forward to it (hell) made humble by disgrace, looking with a stealthy glance... (Surat ash-Shura: 45)

Those who do good will have the best and more! Neither dust nor debasement will cover their faces. They are the Companions of the Garden, remaining in it timelessly, forever. But as for those who have earned bad actions - a bad action will be repaid with one the like of it. Debasement will overshadow them. They will have no one to protect them from God. It is as if their faces were covered with a strip of darkest night. Those are the Companions of the Fire, remaining in it timelessly, forever. (Surah Yunus: 26-27)

As stated in these verses, disbelievers' faces are shadowed by debasement. On the other hand, believers have light on their faces. God states that they are recognised by the traces of prostration on their faces:

Muhammad is the Messenger of God, and those who are with him are fierce to the disbelievers, merciful to one another. You see them bowing and prostrating, seeking God's good favour and His pleasure. Their mark is on their faces, the traces of prostration... (Surat al-Fath: 29)

In other verses, God informs that disbelievers and evildoers are recognised by their faces:

The evildoers will be recognised by their mark and seized by their forelocks and their feet. (Surat ar-Rahman: 41)

If We wished, We would show them to you and you would know them by their mark and know them by their ambivalent speech. God knows your actions. (Surah Muhammad: 30)

It is a miracle and an important secret revealed in the Qur'an that, depending on one's faith or evildoing, physical changes occur on one's face. Spiritual feelings produce physical effects on body; although the contour remains the same, the expression of the face changes or the face is lightened or overshadowed. A man of faith, can, by the Will of God, see this miracle God performs on people.