Imam Saadeq (PBUH) has said:

Any one who believes in all the Infallible Imams and rejects Mahdi, is like he who believes in all Prophets and rejects Mohammad (PBUT).

(Kamaal-O-Ddeen, …)

Let us reflect upon this very profound saying of the sixth Infallible Successor of the Final Holy Prophet (PBUT). All Prophets of Allah came one after the other, delivering the Message of Allah, and they all gave the glad Tidings of the very last Prophet, Muhammad (PBUH&HP); and when he finally came as it was Promised, he clarified all the distortions which had cast their dark shadows upon the beautiful teachings of the previous Prophets (PBUT), and invited the entire humankind to the Right Path of all Divine Prophets. So any one who has responded to his unifying global invitation to everlasting Salvation, must declare two significant matters:

"I bear witness that there is no deity but Allah; I bear witness that Muhammad is the Rasool (Messenger) of Allah."

I.e. if a person claims that he believes in God but he does not believe in all His Chosen Infallible Prophets including the Last Ever-Anticipated Prophet of Allah, Muhammad (PBUH&HP), his belief in Allah will not be accepted from him, for, truly believing in God means obeying Him, and Allah has commanded humankind to believe in all His Prophets, thus believing in all Prophets is an Allah-Defined requisite for acceptance of a person as a Believer. Furthermore, truly believing in the Prophet Mohammad (PBUH&HP) means obeying him, for, Allah in Qur'an has repeatedly commanded the obedience of His Prophet; and the Prophet Mohammad, based upon Allah's command and the verses of Qur'an, has enjoined the belief in and obedience of twelve Infallible Imams after him; Therefore, believing in all twelve Imams (PBUT), including the last of them, he whom we are Anticipating, is an Allah-Specified requisite for acceptance of a person as a Believer.

In these heavy moments of Moharram, entering in to Safar, let us not forget the last of those, whose obedience is the requisite for acceptance of our belief in Allah; let us be at Mahdi's (PBUH) command and ready at all moments for his Emergence and the avenging of the blood of all martyred Prophets and Imams (PBUT); yes, at these mournful moments, let us, with our sad and broken hearts, hope and pray for the sooner occurrence of a world -without oppression, injustice, prejudice, unbelief, deceits, lies, crimes, …- led by the final Imam of the progeny of Prophets (PBUT).