Prophet is always by birth a prophet; prophethood is not cultivable; it is always divine. Let us analyze this viewpoint in light of Quran.
The non-believers used to put forth various objections to Muhammad’s (saw) Prophethood; sura al-anaam has described some of these objections and replies thereto. One objection of non-believers was why whole Quran was not revealed in one go, along with Angels giving testimony about the truth of Quran before the people. This objection has been replied as:
“And even if We had sent down to you, [Muhammad (saw)], a written scripture on a page and they touched it with their hands, the disbelievers would say, "This is not but obvious magic” (al-anaam-7).
Another objection of non-believers and reply thereof has been stated as:
“They say: "Why is not an angel sent down to him?" If we did send down an angel, the matter would be settled at once, and no respite would be granted to them” (al-anaam-8).
This verse states that non-believers used to object why an angel was not sent down to Muhammad (saw); in other words, non-believers were demanding an open miracle and showing their inclination to believe in Angel but not in a human. This objection of non-believers is replied by stating that after happening of an open and glaring miracle like sending down an Angel, there would be no time left for the non-believers to put forth any other lame excuse; if they did not believe in Muhammad (saw), even after such a glaring miracle, they would be punished immediately after that, and they would not be granted any respite. Another reply to this objection of non-believers is given as following:
“If We had made him (our Prophet) an angel, We would have made him a man, and We would certainly have caused them confusion in a matter which they have already covered with confusion” (al-anaam-9).
In this verse Allah is stating that if He made an angel His Prophet, even then He would have made him a man. It means Allah first appoints anyone his Prophet and after that He determines/ fixes his sex, race and specie. If any angel is to be appointed as prophet, he is first appointed as prophet and after that his specie is fixed- the specie in which he is to be born. In other words, a prophet is appointed as prophet even before his birth. The possibility that an angel may also be made a prophet for humans, implies that a prophet is always infallible. The possibility that an angel may also be made a prophet for humans also implies that prophet-hood is not something cultivable; it cannot be earned through a process of committing mistakes and sins.
We may conclude that a prophet is always by birth a prophet.