Your question concerning the reappearance of the Imam of Time can be investigated around four focal points:

First: The signs for his coming are categorized into different groups, one of them being their classification into “certain” and “uncertain” signs: According to some hadiths, five signs are considered “certain” which we will mention them cursorily:[1]

Abu Hamzah Thumali says: “I told Imam Jafar Sadiq (as) that his father, Imam Muhammad Baqir (as) once said: “The emergence of an individual by the name of Sufyani is for sure, the echo of a call from the skies is also for sure, as is the rise of the sun from the west,” he also named other signs that are also for sure.”

Imam Jafar Sadiq (as) completed his father’s words by saying: “In addition to the internal conflict between Bani “Felan” (felan in Arabic means “someone” and is used when the person speaking doesn’t want to directly name someone and here what might be meant by His Excellency is the tribe of Bani Umayyah or maybe the army of evil), the killing of the Nafs Zakiyyah (the pure one), and the emergence and rise of the Qa’im (Imam Mahdi (as)) which in reality is a vow that will without a doubt take place. I (Abu Hamzah) asked: “Can you please explain about the cry from the sky?!” He answered: “In the beginning of a certain day, a caller will call in such a way that each nation will understand what is being said through its own language, saying: “All be aware that the truth is in Ali (as) and his followers!”, and at the end of the day, Iblis (Shaytan) will call out saying: “All be aware that the truth lies in the Sufyani and his followers!”

Here, those who don’t have stable faiths will begin to doubt.”[2], [3]

Second: In order that a person should remain steadfast and have a lasting belief in Imam Mahdi (atf) and to remain unflinchingly adherent to his belief at the end of time, he needs to fight his carnal desires, remember God and trust in God, resort to the Quran and the Ahlul-Bayt (a.s), deepen his relationship with Ahluz-Zikr (people of remembrance) or jurisprudents who are the trustees of the infallible Imams and their successors. That is because during the period of occultation, Satanic temptations and specious arguments increase and the growing material attractions influence and encompass the lives of the entire people. Meanwhile, the number of people who are steadfast in their loyalty to the Promised Mahdi (atf) and the reappearance of the Savior of the World is so few and rare that they, as reported in traditions, have been likened to alchemy:

"By the One Who sent me with the Truth as a Herald of Glad Tidings, those who will remain steadfast in the belief in him in the time of his occultation, they are rarer than alchemy.”[4]

Allamah Amini says in his book, Mahdi, the Just Leader of Humanity:

During the occultation, innovations will appear in the religion, and the ordinances of the Qur'an and the teachings of Islam will be interpreted in accordance with people's likes and dislikes. As a result, many teachings and laws will be forgotten as if they were never even a part of Islam.[5]

Different readings of the religion are the most dangerous means of assault which we face during the period of occultation. When the Imam of Time (atf) reappears, he shall come across some so-called scholars (not idolaters) who interpret the Quran according to their personal reason or opinions (ra'y). Eventually, different readings of the religion become so widespread that all people turn to believe in them.[6]

People, having abandoned the absolute principles and fundamental teachings of Islam, merely follow the outward forms of religion, and regard those to be sufficient. These are the people who, besides the five daily obligatory prayers, the fasting of Ramadan, and the avoidance of external pollution (najasat), know nothing of Islam. They do not regard ethical behavior and moral precepts to be part of Islam. They give no importance to eschewing immoral conduct and make an excuse of not following moral guidelines since there is dispute about the obligatoriness and the prohibitions of certain requirements. They go as far as turning the prohibitions of the law, through trickery, into something permissible. They also shun their responsibility for paying the dues that are imposed by the law on them. In other words, they are engaged in observing the religion according to their desires. Certainly, such a religion would seem new and difficult to these people, and they might not even consider it to be Islam, because they have imagined Islam to be something else.[7],[8]

Thus, with this wrong assumption, a great number of people and even a group of so-called scholars who are apparently religious-minded people stand up against the Imam, reject the truth and real Islam as false and consider their own religion and beliefs as true. In this manner, they deviate from the right path and become disbelievers.

It has been narrated from Imam Sadiq (a.s.) that he said: "When our Qa'im arises he will call people anew to Islam, guiding them to the old thing from which people have turned away."[9]

Third: According to traditions, following the reappearance of the Hujjah of God (Imam Mahdi), Prophet Jesus will offer prayers behind him in the presence of others. It has been narrated from the Prophet (pbuh) that he said: "By Allah Who appointed me as a bearer of good news, if there was one day left of this world, Allah will prolong that day so that my son, Mahdi appears. Afterwards Jesus, the Spirit of Allah, shall come and will perform his prayer standing behind him. Then the earth will brighten up with the Light of God and Mahdi's government shall spread to the east and west of the universes."[10]

Since a great many people make the followers of Prophet Jesus (a.s) and he regard Mahdi (atf) as the savior and global reformer, they, seeing this scene get interested in Mahdi (atf) and start to believe in him. This by itself is one of the causes of the victory of the Imam (atf) in his global movement and rise.

Fourth: In the same way that the Holy Prophet (pbuh) is the last prophet and his religion is the most perfect of all religions, Imam Mahdi is also the last infallible Imam (atf) who with his qiyam (rise) shall bring a great change in the world and shall help mankind attain the peak of perfection, knowledge and the highest level of piety, spirituality and justice.

In regard to the perfection of knowledge, Imam Sadiq (a.s) says: “Knowledge is contained in twenty-seven letters (parts) and everything which all of the (previous) messengers brought was merely two of these letters and until the day (when al-Mahdi makes his advent), the people will not be given knowledge except of these two letters. So then when our Qa’im makes his advent, he will bring with him the other twenty-five letters (of knowledge) and he will spread these among the people and add these to the previous two letters (parts) such that he will have imparted the twenty-seven letters (of knowledge) to them.”[11]

If we assume that "letter" refers to a discipline or field of knowledge, then it means that all the sciences and knowledge that mankind will achieve or are available to them till the advent of the Imam will be equal to two disciplines out of 27 disciplines. The remaining 25 disciplines will be revealed to humans by the Imam of Time (atf) during the period of occultation. [12]