Every verse in the Qur'an addressing to the reign of Islamic morality refers to the system of the Mahdi. God has promised in the verses that Islamic morality will prevail. Our Prophet (saas) reported that this will be ensured by means of Hazrat Mahdi (pbuh) and described in detail how peace, love, wealth and enlightenment will prevail in the world during this period. The 55th Verse of Surat An-Nur is one of those verses in the Qur'an that herald the reign of the Islamic morality and the system of the Mahdi.
God has promised those of you who have faith and do right actions that He will make them successors in the land as He made those before them successors, and will firmly establish for them their religion with which He is pleased and give them, in place of their fear, security... (Surat An-Nur, 55)
WaAAada Allahu allatheena amanoo minkum waAAamiloo alssalihati layastakhlifannahum fee al-ardi kama istakhlafa allatheena min qablihim walayumakkinanna lahum deenahumu allathee irtada lahum walayubaddilannahum min baAAdi khawfihim amnan yaAAbudoonanee layushrikoona bee shay-an waman kafara baAAda thalika faola-ika humu alfasiqoona
WaAAada Allahu: God has promised
As stated in many verses of the Qur'an, the promise of God is true, and God never breaks His promise.
fee al-ardi: in the land
This refers to the prevalence of Islamic morality all over the land, all over the world without any exception.
"Certainly, that act (the prevalence of this religion) will extend over where ever the day and the night reach across. God will not leave a brick house or a felt tent; by canonizing saints and condemning despicable ones, will make them follow this religion. What God will canonize is Islam; what God will condemn is blasphemy." (Ahmed b. Hanbal - Tabarani - Ibn Hibban)
layastakhlifannahum: He will make them successors
The lexical meaning of the word 'successor' (caliph): Acknowledging God as "the Absolute and Sole Owner of Sovereignty" and exercising the power and the authority granted by Him in accordance with the morality commanded by Him.
This phrase is used in the verse so as to imply the sovereignty, superiority, victory and rule of the devout believers. God has promised to make them successors (caliphs) in the land so that they can establish the moral values that Almighty God pleases, implement the order God commands based on love and brotherhood and raise mankind to the level of maturity that He has ordained for them ever since the day Almighty God created them.
He will make them successors in the land as He made those before them successors
That He will make them come to power, in other words, and enable them to have power and safety, thereby, to rule the whole world.
walayumakkinanna lahum deenahumu allathee irtada:
He has promised that He will firmly establish for them their religion with which He is pleased.
The firm establishment of the religion on the Earth and in people's hearts is only possible when everyone acts in accordance with this good morality. In this manner, Almighty God has promised to make believers successors in the land and to firmly establish for them their religion with which He is pleased.
As it can be seen, the verse refers to the prevalence of Islamic morality in the world and a period in which people will live in the beauty of Islamic morality. That promise will be fulfilled in the time of Mahdi (pbuh), in other words, in this century. When it is fulfilled, Hazrat Mahdi (pbuh) will be the spiritual leader and pioneer of Muslims.