And regarding the Master of Our Time عليه السلام, Allah the Almighty said: "On the Day when some of the signs of your Lord come, no soul will benefit from its faith if it had not believed before." (6:158) Al-Sadiq Jaʿfar b. Muhammad عليه السلام was asked about this verse, and he said: "The signs are the Imams, and the awaited sign is al-Qaʾim al-Mahdi عليه السلام. So when he rises, no soul will benefit from its faith if it had not believed before his rising with the sword, even if it had believed in those who preceded him from among his forefathers عليه السلام."
This was narrated to us by Ahmad b. Ziyad b. Jaʿfar al-Hamadani (may Allah be pleased with him), who said: ʿAli b. Ibrahim narrated to us from his father, from Muhammad b. Abi ʿUmayr and al-Hasan b. Mahbub, from ʿAli b. Riʾab and others, from al-Sadiq Jaʿfar b. Muhammad عليه السلام.
[Al-Saduq says:] This is proved by the Almighty Allah in this verse: And We made the son of Maryam and his mother a sign. (23:50) It means the Almighty Allah has made him the proof and he says regarding death of Prophet Uzair for a hundred years and making him alive again: And look at your ass; and that We may make you a sign to men. (2:259) Here also he wanted to make him proof on the people. Thus the Almighty Allah made him proof on the people and called him a sign.