Why should Muhammad be the Final Prophet? Haven't there been others after him?

Prophethood is not something acquired by a person who proves himself worthy, nor is it granted in recognition of piety. Prophethood is an office to which god appoints a man in order to fill a particular need. the Qur'an mentions four conditions under which prophets were sent to the world:

1. When no prophet had ever been sent to a people before and no divine message had reached them
2. when the message of an earlier prophet had been forgotten by the people or the teachings of former prophets had been altered with time
3. when a second prophet was needed to assist a first one
4. when a people had not yet received complete instruction from god

In each of these cases a prophet was appointed to convey divine revelation, updating previous messages and correcting deviations that man had introduced into the religion of god.

After god's message was completed through revelation to Muhammad and its preservation guaranteed, there was no further need for messengers to convey revelation; only for teachers and reformers to remind people of what god had revealed.[1] from the time of Muhammad's Prophethood, conditions in the world have been conducive to the transmission of god's message to all civilizations, making the appointment of additional prophets unnecessary. this final message has undergone no amendment or alteration by man; not a single word has been added to it or deleted from it.

If god intended to send another prophet after Muhammad, he would have made that fact clear in the Qur'an or commanded his messenger to declare that a prophet would follow him. but the Qur'an clearly affirms that god has now completed his divine mission through prophet Muhammad.[2] therefore, the office of Prophethood has been canceled, enabling the world to unite in allegiance to the final prophet and obedience to god. for everyone who accepts Muhammad as the divinely appointed final messenger will seek instruction only within the message he conveyed.

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Footnotes:

[1]  the Qur’an states: “Muhammad is not the father of [any] one of your men, but [he is] the messenger of Allaah and seal [i.e., last] of the prophets.” (33:40)

[2]  god stated therein: “this day i have perfected for you your religion and completed my favor upon you and have approved for you Islam as religion.” (5:3)