Belief in the Messengers implies: 1- Firm belief that Allah sent to every nation a messenger to call them to worship Allah Alone, 2- Belief in those messengers whose names we know, such as Muhammad, Ibrahim, Musa, ‘Isa and Nuh, 3- Believing in the sound reports that have been narrated from the messengers, and 4- Following the laws of the messenger who has been sent to us, namely the Final Prophet, Muhammad, who was sent to all of mankind.

We need prophets to guide us to that which is best for us in this world and in the Hereafter. We cannot follow what is best for us with regard to the Hereafter unless we follow the message. We cannot be guided to what is best for us in this world unless we follow the message.

From the general meaning of the Quranic verses “Then they found one of Our slaves, on whom We had bestowed mercy from Us, and whom We had taught knowledge from Us” [al-Kahf 18:65], it appears that Al-Khidr was a prophet.

Prophet Adam, the father of mankind, was a Prophet, so he was the first of the Prophets and Prophet Nuh was the first of the Messengers of Allah.

Prophet Noah was one of the messengers of Allah. Noah called his people, and he remained among them for one thousand years less fifty (i.e., 950 years), calling them to worship Allah Alone and to give up idol-worship. For more, see the detailed answer.