All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
If the one who said such words means that religion has already been perfected and that the Prophet, sallallaahu ‘alayhi wa sallam, has delivered the Message of Allaah, fulfilled his role in the best way, ordered us to do all things that can bring us closer to Allaah The Almighty, and forbade us from doing all things that can keep us away from Allaah The Almighty, then that is absolutely right. Religion has been already perfected and the Prophet, sallallaahu ‘alayhi wa sallam, has delivered the message in the best way. Thus, religion no longer needs the Prophet, sallallaahu ‘alayhi wa sallam.
Allaah The Almighty Says (what means): {This day I Have Perfected for you your religion and Completed My favor upon you and Have Approved for you Islam as religion.}[Quran 5:3]
However, if he meant undermining the role of the Prophet, sallallaahu ‘alayhi wa sallam, and that the right Religion of Allaah The Almighty did not need the Messenger, sallallaahu ‘alayhi wa sallam, to convey and explain the Message of Allaah The Almighty to the people, and that the people have been guided to Islam by nature and without any explanation from the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, then first he would be telling a lie and secondly he would become a non-Muslim, we seek refuge with Allaah The Almighty from this. Accordingly, he should return to Allaah The Almighty in repentance immediately, pronounce the two testimonies of faith, perform Ghusl and never return to such thing again.
Scholars of the Hanafi School of Fiqh said that if someone said that the Prophet, sallallaahu ‘alayhi wa sallam, was an illiterate with the intention of belittling him, he would be a disbeliever. Shaykh-ul-Islam Ibn Taymiyyah, May Allaah have mercy upon him, said, “Is-haaq ibn Raahawayh said that Muslims have agreed unanimously that whoever insults Allaah The Almighty or His Messenger, sallallaahu ‘alayhi wa sallam, rejects anything of what Allaah The Almighty Has Sent down, or kills one of the Prophets of Allaah, he will be a disbeliever even if he acknowledges all that Allaah The Almighty Has Sent down."