The Ahle Tasannun scholar Shaikh Sulaiman Qundoozi, in his book Yanaabee’ al-Mawaddah (chapter 71, p. 485), narrates a tradition from Imam Ja’far Sadiq (peace be upon him). In this narration, Mufazzal Ibn Umar enquired from Imam Ja’far Sadiq (a.s.) about Surah Shura (42): Verse 18
إِنَّ الَّذينَ يُمارُونَ فِي السَّاعَةِ لَفي ضَلالٍ بَعيدٍ
".Those who dispute obstinately concerning the Hour are in great error"
Imam (peace be upon him) replied: “The skeptics ask, 'When was Imam Mahdi (a.t.f.s.) born? Who has seen him? Where is he now? When will he reappear?' All of this is akin to doubting the Divine Decree and Power. These are those who will be at a loss in this world and the hereafter.”
Reference: Yanaabee’ al-Mawaddah, chapter 71, p. 485
To summarize, it is clear from both the Quranic verse and the tradition of Imam Sadiq (peace be upon him) that people will doubt the existence of Imam Mahdi (a.t.f.s.). Not only will they be in doubt themselves, but they will also create doubts for others. According to this tradition, doubting the existence of Imam Mahdi (a.t.f.s.) is equivalent to doubting the Decree and Power of Allah.
The sending of 124,000 Prophets (a.s.), the establishment of Islam as the final Shariah, and the setting of the Day of Judgment are all Divine Decrees; this means that these are Allah’s decisions in which no one can interfere, as Allah has established them as certainties.
Similarly, the entire universe was created by Divine Power, and Allah can do whatever He wills, whenever He wills. He possesses the power of the command, "Be!" and it is (36:82), and He has power over all things. One who doubts Allah’s Decree and Power has, in reality, cast doubt on Allah Himself.
In the same way, the existence of Imam Mahdi (a.t.f.s.), his reappearance, and his rule are the Decree and Power of Allah. Therefore, anyone who has doubts concerning Imam Mahdi (a.t.f.s.) has undermined Allah’s Power, and such a person will be at a loss in this world and the Hereafter.
Lastly, we pray to Allah that He hasten the reappearance of Imam Mahdi (a.t.f.s.) and protect us from the evil of those who are skeptical of him.
Aameen, O Lord of the worlds!