Al-Hadid (The Iron)

12. On the Day you shall see the believing men and the believing women: their light running forward before them and by their right hands. [It is said unto them:] Glad tidings for you this Day! Gardens under which rivers flow, to dwell therein forever! Verily, this is the great success.

The light of faith leads believers to Paradise on the Day of Resurrection.

Thus, the blessed Verse is saying:

"O Muhammad! Remember the Day when you see believing men and women whose light forwards before them and on their right hand side bearing them the glad tidings of being admitted to Paradise. Such light surrounding believers is that of faith, monotheism, and righteous good deeds. It is the spiritual light of belief and certitude in Divine Origin and Resurrection shedding light before them and that of righteous good deeds on their right hand side. In the same manner that faith serves as man's guide to happiness and spiritual and ethical virtues, it will shed light when the world will be plunged into shadows. Such light, emanated inwardly, is the light of monotheism and certitude possessed by those near unto Divine Threshold. They are the people on the right. The intensity of the light depends upon faith, righteous good deeds, certitude, and fear of God Almighty.”

It is also reflected in traditions that the light of the good differs in terms of luminescence, namely, any God fearing believer will shed light before him and on his right depending the degree of his faith.

Thus, believers will be led to Paradise, the abode of Divine Grace, to dwell therein everlastingly. Angels of Mercy shall welcome them bearing them glad tidings of gardens under whose trees and arbors rivers of Divine Infinite Emanation and Bounties flow.

The best tidings for the people of Paradise are that they shall dwell therein forever.

"This is the great success"

reflects Divine Favors in store for the people of Paradise. Such happiness and merit shall be bestowed upon believers.