In the name of Allah, the Most Gracious, the Most Merciful.
WHY ALLAH SENT PROPHETS AND MESSENGERS? (AS)
Ever since people innovated the dogma of Shirk (i.e. joining others in worship along with Allah), Allah had been sending Prophets and Messengers to his devotees in order to invite them to the worship of Allah and Allah alone, to order them not to ascribe partners to Him and bring them out of darkness of polytheism into the light of Monotheism. All the Prophets preached Tauhid (i.e. Monotheism, the Belief in the Oneness of Allah, the Glorious, the Elevated). The following verses from the Noble Quran illustrate this fact:
“Indeed We sent Nuh (Noah) to his people, and he said: ‘O my people! Worship Allah! You have no other Ilah (God) but Him. (La illaha illallah, none has the right to be worshipped but Allah). Certainly, I fear for you the torment of the great Day!” (V. 7:59)
“And to ‘Ad (people, We sent) their brother Hud. He said: ‘O my people! Worship Allah! You have no other Ilah (God) but Him. (La illaha illallah, none has the right to be worshipped but Allah). Will you not fear (Allah)?” (V. 7:65)
“And to (the people of) Madyan, (We sent) their brother Shuaib. He said: O my people! Worship Allah you have no other Ilah (God) but Him. (La illaha illallah, none has the right to be worshipped but Allah). Verily, a clear proof (sign) from your Lord has come to you, so give full measure and full weight and wrong not men in their things, and do not make mischief on the earth after it has been set in order, that will be better for you, if you are believers.” (V. 7:85)
“And to Thamud (people, We sent)their brother Salih. He said: ’O my people! Worship Allah! You have no other Ilah (God) but Him. (La illaha illallah, none has the right to be worshipped but Allah). (V. 7:73)
“And verily, We have sent among every Ummah (community, nation) a Messenger (proclaiming): Worship Allah (Alone) and avoid (or keep away from) Taghut (all false deities, i.e. do not worship Taghut besides Allah).” (V. 16:36)
Every Prophet was sent to his own nation for their guidance, but the message of Prophet Muhammad SAW was general for all mankind and jinn. As in Surat al A’raf (Allah addresses His Messenger SAW):
“Say (O Muhammad SAW): ‘O mankind! Verily, I am sent to you all as the messenger of Allah.’ ” (V. 7:158)
So the aim of sending these Prophets and Messengers to mankind and jinn was only that they should worship Allah Alone as Allah SWT said:
“And I (Allah) created not the jinn and mankind except that they should worship Me (Alone)” (V. 51:56)
And to worship Allah means to obey Him and to do all that He has ordained, – and to fear Him by abstaining from all He has forbidden.
Then those who obey Allah will be rewarded in Paradise, but those who will disobey Him will be punished in Hell-fire.