God’s messengers always named a successor to guide the people after they are gone. This is clearly found in the Torah, the Bible and the Quran. Prophet Muhammad (pbuhahf) did exactly the same thing, leaving a final Will and naming in it the divine guides who would follow him.
“O Father of Al-Hassan, bring me a pen and a paper’, so the Messenger of Allah (pbuhahf) dictated his Will until he came to this position where he said: ‘O Ali, there will be twelve Imams after me and after them there will be twelve Mahdis. So you, O Ali, are the first of the twelve Imams, Allah The Exalted has named you in His heavens Ali Al-Mortadha (Ali the Content), Amirul Mo’mineen (the Commander of the Believers), Al-Siddiq Al-Akbar (the Greater Truthful), Al-Farouq Al-A’tham (the Greater Judge and Differentiator between truth and falsehood), Al-Ma’moun (the Trusted), and the Mahdi (the Rightly Guided). These names may not be attributed to other than you. O Ali, you are my Vicegerent/Guardian on my own family, their living and their dead, and upon my women: Whomever I kept, she shall find me tomorrow, and whomever I divorced, I am innocent of her, I will not see her and she will not see me on the Day of Resurrection. And you are my Successor (Khalifa) on my nation after me. If death comes to you, hand it over to my son Al-Hassan, the Righteous and Benevolent. Then if death comes to him, let him hand it over to my son Al-Hussein, the Martyr, the Pure and Murdered. If death comes to him, let him hand it over to his son, the Master of Worshippers, Dhul Thafant (the one with hard skin on his knees) Ali. If death comes to him, let him hand it over to his son, Muhammad Al-Baqir (the Revealer of Knowledge). If death comes to him, let him hand it over to his son, Ja’far Al-Sadiq (the Truthful). If death comes to him, let him hand it over to his son, Musa Al-Kadhim (the Patient). If death comes to him, let him hand it over to his son, Ali Al-Ridha. If death comes to him, let him hand it over to his son, Muhammad Al-Thiqa Al-Taqqi (the Trustworthy, the God-Fearing). If death comes to him, let him hand it over to his son, Ali Al-Nasih (the Advisor). If death comes to him, let him hand it over to his son, Al-Hassan Al-Fadhil (the Meritorious). If death comes to him, let him hand it over to his son, Muhammad the Safeguarded of the Family of Muhammad, peace be upon them. Those are the twelve Imams. Then there will be twelve Mahdis after him, so if death comes to him, let him hand it over to his son, the first of the close ones, he has three names, one like mine and my father’s and it is Abdullah (Servant of God), and Ahmad (the Praised), and the third name is Al-Mahdi (the Guided) and he is the first of the believers.”
– Al-Ghayba, Muhammad ibn Al-Hassan ‘Al-Toosi’, 3rd ed., p. 150, hadith 111
The Will of Prophet Muhammad (pbuhahf) was written on the night of his death, and Prophet Muhammad (pbuhahf) described his Will as being a “protection from misguidance” for whoever holds tight to it. This is mentioned in the most authentic books of all Muslims, both Sunnis and Shias.
The fact that the Prophet (pbuhahf) describes this Will as being a “protection from misguidance” This means that it is impossible for anyone to claim this Holy Will except for its rightful owner. The Messenger of Allah (pbuhahf) does not speak of his own desire, rather it is an inspiration given to him by God Almighty, as mentioned in the Holy Quran.
“Your companion [i.e. Muḥammad] has not strayed, nor has he erred, Nor does he speak from [his own] inclination. It is not but a revelation revealed.”
– Holy Quran, Al-Najm verses 2-4
Whoever argues that any false claimant can claim the Will is accusing God Almighty of misguiding the people, after they followed the Will which the Prophet clearly described as a “protection from misguidance”! Far away is God Almighty from that. Rather, He SWT must protect and safeguard this Holy Will until the aim from it is fulfilled, and this is clearly stated in the Holy Quran:
“And if he had invented false sayings concerning us (in Our name), we would have seized him by the right hand. Then certainly We would have cut off his life artery.”
– Holy Quran, Al-Haqqa verses 44-46
However, there have been many false claimants who have claimed that they were messengers from God or prophets of God or that they were the Mahdi, and their life was not cut off by God. So what does God Almighty mean by the Holy verse mentioned above?
He means that He SWT would cut off the life of the ones who falsely claim something which He had promised to be a protection from misguidance and which he has guaranteed as a proof upon the people, like the Will of Prophet Muhammad (pbuhahf). God would cut their life off (meaning make them die) before they even make their claim public, because God had promised that holding tight to this Will is a protection from misguidance so He would not allow anyone to claim it falsely.
So anyone can falsely claim to be the Mahdi, or a prophet of God, and it happened several times in the past. But no one ever claimed to be “Ahmad” mentioned in the Holy Will of Prophet Muhammad (pbuhahf) except for Ahmad Al-Hassan (From Him is Peace) who was the first to ever claim that.
And as such, no one ever claimed to be Abdullah mentioned in the Holy Will of Prophet Muhammad (pbuhahf) except for Abdullah Aba Al-Sadiq (From Him is Peace), who did so by the command of Imam Ahmad Al-Hassan (From Him is Peace) the leader of the Black Banners of the East.
Thus, according to the Holy Quran and according to the hadiths of the Ahlulbayt (pbut), it is impossible that anyone can be the first to claim a divine text (like the Holy Will of Prophet Muhammad), except its rightful owner.
Therefore, Imam Ahmad Al-Hassan (From Him is Peace) is indeed “Ahmad” who is mentioned in the Holy Will, and indeed Abdullah Aba Al-Sadiq (From Him is Peace) is “Abdullah” who is mentioned in the Holy Will.
Hence, they are the ones whom holding tight to protects from misguidance and leads to the one true God Almighty.