Once my friend was discussing the creed of the Christians with his father. Then he said, while he was angry, do you believe that their religion and Prophet are really true? He regretted saying such words in the same gathering. Then, he said to his father that he meant that he [‘Eesa (Jesus)] is not the seal of the prophets and not the prophet that should be followed now.
Is this a form of disbelief in the prophets (while he did not mean what he uttered)? Is this a kind of the disbelief that renders one an apostate? Will it be obligatory for him to perform Ghusl and pronounce the Two testimonies of faith again or repentance will be enough?

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.

Belief in the messengers, may Allaah exalt their mention, and honoring and regarding them highly are among the pillars of the sound faith. Allaah The Almighty Says (what means): {Say, [O believers], "We have believed in Allaah and what has been revealed to us and what has been revealed to Ibraaheem (Abraham) and Ismaa‘eel (Ishmael) and Is-haaq (Isaac) and Ya‘qoob (Jacob) and the Descendants and what was given to Moosa (Moses) and ‘Eesa (Jesus) and what was given to the prophets from their Lord. We make no distinction between any of them, and we are Muslims [in submission] to Him.}[Quran 2:136]

Additionally, scholars have agreed unanimously that whoever insults, disrespects or belittles any one of the prophets, may Allaah exalt their mention, will be a Kaafir (disbeliever). Is-haaq ibn Raahawayh said, “Muslims have agreed unanimously that whoever insults Allaah The Almighty or one of His messengers, rejects anything of what Allaah The Almighty Has Sent down, or kills one of the Prophets of Allaah, he will be a Kaafir even if he acknowledges all what Allaah The Almighty Has Sent down (i.e. Has Revealed)."

As for his saying (he did not mean what he uttered), whoever did something of Kufr (disbelief) or spoke of it, he would be a Kaafir even if he did not mean it. Ibn Taymiyyah said, “Whoever spoke something of Kufr (disbelief) or did one of its actions, would be a Kaafir even if he did not mean it, for only just few individuals may intend Kufr except what Allaah The Almighty Wills."

Muhammad ibn Hazm said, “Whoever uttered a word of Kufr and Muslims deemed him as a Kaafir he would be a Kaafir. However, if he just read, witnessed, narrated it, or was compelled to say it, he would not be deemed so."

Accordingly, what this person uttered apparently includes denying the Prophethood of ‘Eesa and this is a form of Major Kufr that gets one out of Islam. If this person meant this, he would be a Kaafir and he would have to turn to Allaah The Almighty in sincere repentance, pronounce the Two testimonies of faith, and perform Ghusl according to some scholars. However, if he did not mean what he said, but he actually intended the other meaning he mentioned, he would not be a Kaafir and should observe that he did was an evil thing while speaking in this way. Also, he has to avoid uttering such words again, repent sincerely, and ask for the Forgiveness of Allaah frequently and perform many good deeds.

Allaah Knows best.