Imam Khomeini said on various occasions as following: 

Imām Mahdī (PBUH) is assigned to set right all these perversions and deviations. He will bring man from the abyss of depravation. It is said: “He fills the earth with justice after having been replete with tyranny.” (  Sahifeh-ye-Imam, Vol.12, page 409)

How auspicious is the birthday of the one who is to establish universal justice, for which purpose all the divine prophets (peace be upon them) have been sent!  (Sahifeh-ye-Imam, Vol.14, page 402)

Then he will reappear and the world will be filled with justice at a time when there is injustice everywhere and that is why he is now hidden from the sight of men. And when he comes with his back towards the Ka'bah's edifice,

Amir al-Mu'minin said: "Our Qa'im has an occultation which will be of great devotion... Beware! Everyone who is firm in his religion, and does not become hard-hearted from the lengthening of the occultation of this man (and turn away from his religion) will find himself in the same rank as me on the Day of Resurrection."

 Then he said: "Our Qa'im, when he rises, has no responsibility for anyone's allegiance to him, and for this reason his birth will be secret and he will be hidden." 3. Muhammad ibn Muslim said that he heard from Imam Ja'far as-Sadiq (A.S.) that he said: "If you hear of the occultation of your Imam, do not deny it." 4. At-Tabarsi wrote: "The hadiths of the occultation were recorded by the Shi'ite narrators in books which were composed in the time of Imam Baqir and Imam Sadiq (A.S.). 

The belief in the reappearance of the expected Mahdi (A.S.) the universal reformer is not confined to the Shi'ite Muslims. Other Islamic groups and even non-Islamic groups like the Jews and the Christians and some of the great world intellectual figures believe in the appearance of a great spiritual reformer. In Psalm 37 is written: "...Trust in the Lord and do good; so you will dwell in the land, and enjoy security. ...For the wicked shall be cut off: but those who wait for the Lord shall possess the land. ...But the weak shall possess the land, and delight themselves in abundant prosperity.

...The Lord knows the days of the flawless, and their heritage will abide forever. ...For those blessed by the Lord, shall possess the land, but those cursed by Him shall be cut off. ...The righteous shall possess the land, and dwell upon it at peace for ever..."

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