In the year 260, Imam Mahdi (a.s.), by the order of God, began the stage of disappearance after completing the ceremonial prayer for the funeral of his father. Therefore, people were no longer able to present their problems or questions directly to the Imam. From the year of 260 to the year of 329 Hijri, four specific and special Deputies, or Naebs, were set to send the messages to and from the Imam between him and the people. This period of 69 years is known to be the Minor Occultation (Ghaybah Sughra). The Vicegerents came in sequential order. These humble people were known to be the Four Deputies (Nowaabeh Arba'eh) the first of which was Othman ibn Saeed Amri. The second, his son, was Muhammad ibn Othman Saeed Amri. After him came Hussein ibn Rouh Nawbakhti, and the last being Ali ibn Muhammad Sammari. One week before the death of the fourth and last Deputy, there was a special letter, issued by Imam Mahdi (a.s.) to Sammari himself, stating: “Ya Ali ibn Muhammad, you will pass away from this world within a week; do not assign anyone after yourself, to the position that you have been assigned. It is because this position will no longer be available, and after your death, the Major Occultation (Ghaybah Kubra) will initiate.” Once the fourth Deputy had passed away, the Major Occultation began. Since the era of the Major Occultation began, we Shias pray for the reappearance of the Imam, who will bring upon the world absolute peace, prosperity and justice.
Meaning & purpose of the Occultation
The Occultation of Imam Mahdi (a.s.) means that he either is isolated from the rest of the world's population, or he lives among them and as stated in hadith, "He walks in the markets, sees the people and knows them, yet they see him and do not recognize him."
The purposes of his disappearance, as partially stated in hadiths, include safeguarding him from his enemies, freeing him of any obligation before any government – which for years hindered the progress of a number of his predecessors – and giving him the means of preparation for his grand and glorious responsibility to govern the whole world and bring peace and unity to mankind. More concise reasons of the Imam's disappearance, as mentioned in hadiths, will be better understood once he reappears.
It is noteworthy to say that “Occultation” or disappearance has roots in the Quran, where it is stated that belief in this concept is a provision of piety.
Longevity
The longevity of Imam Mahdi (a.s.) may raise questions for some people. How do we celebrate his 1178th birthday now? Is it possible for someone to live that long? The responses are:
1- God, who has created all, is capable of doing anything he desires. Though some of it may be unclear to us, through reasons he seems fit, he has given the order of the longevity of our savior, so that he may appear at the right time.
2- Life is a principle; life is the basis; while death is adventitious. You never ask anyone why he or she is alive. You do not ask for a reason why people live, while death needs a cause. When you hear that someone has passed away, the first questions that come to mind are: "Why?" "By what reason?" and "How?" So, death is an effect. If someone knowingly avoids the causes resulting in death, he or she can certainly live as long as possible. Imam Mahdi (a.s.), who is the last Godly resource, has been given such knowledge by Almighty Allah, and is well aware of how to avoid such issues, and is protected by Allah as well, so, he can live as long as needed.
3- Examples of longevity among many different divine representatives in the past indicate that long life is not unusual. For instance, prophet Noah lived 2500 years. As the Quran states, he was prophet for his people for 950 years before the flood. Among more recent examples, we know Salman Farsi, who lived for 400 years.
According to the Quran (4:159) Prophet Jesus is still alive, and the belief of his existence is a part of the basic principles of Islam. Imam Mahdi (a.s.) as hadiths say, has similarities to different prophets; his similarity to Prophet Noah is his longevity. Also, hadiths educate us that in spite of his long life, the Imam has a young face, resembling that of someone less than 40 years of age. Oh Allah, expedite the re-appearance of Imam Mahdi (a.s.); give us the chance to better serve him and enable us to perform our duties appropriately.