A - Worshipping Allaah Alone:
This is the reason behind Allaah's creating all that exists: Angels, Jinn, mankind, heavens, earth and animals etc. Allaah created everything only to worship Him alone. He Said, (what means): "See you not that to Allaah prostrates whoever is in the heavens and whoever is on the earth, and the sun, and the moon, and the stars, and the mountains, and the trees, and the animals, and many of mankind? But there are many (men) on whom the punishment is justified. And whomsoever Allaah disgrace, none can honor him. Verily! Allaah does what He will." [Quran 28:18]
He also said about the Angels, (what means): "Surely, those who are with your Lord (angels) are never too proud to perform acts of worship to Him, but they glorify His praise and prostrate before Him." [Quran 7:206]. And (what means): "There is not a thing but glorifies His praise. But you understand not their glorification." [Quran17:44]
There are many other verses with this meaning.
Allaah sent the Prophets, may Allaah exalt their mention, and sent down revelations to them for the above reasons. He said, (what means): "And verily, We have sent among every nation a Messenger (proclaiming): "Worship Allaah (Alone), and avoid Taghoot (all false deities)." [Quran 16:36]
Prophet Muhammad , was the last Messenger sent by Allaah to all nations, with the goal of worshipping Allaah Alone. He commanded people to worship Allaah Alone, taking no partners with Him in worship.
B - Establishing Allaah's Proof:
Allaah's Sunnah (way) with regard to His creation is that not all humans, rather, only a few of them are rightly guided. The most important role for the Messenger, sallallaahu ‘alayhi wa sallam, is to warn the disbelievers and rebellious Muslims. Allaah Says, (what means): "Verily, We have inspired you (O Muhammad) as We inspired Noah and the Prophets after him; We (also) inspired Abraham, Ishmael, Isaac, Jacob, And al-Asbat (the twelve sons of Jacob), Jesus, Job, Jonah, Aaron, and Solomon, and to David We gave the Psalms. And Messengers We have mentioned to you before, and Messengers We have not mentioned to you, and to Moses Allaah spoke directly. Messengers are bearers of good news as well as of warning in order that mankind should have no plea against Allaah after the Messengers. And Allaah is Ever All-Powerful, All-Wise." [Quran 4:163-165]
What Allaah meant by (what means): "In order that mankind should have no plea against Allaah after the Messengers" [Quran 4: 163-165] is that, on the Day of Judgment, they will not be able to use the excuse of not having a warner from Allaah during their lifetimes. This excuse will not justify their continuing disbelief. The fact that Allaah has sent the Messengers, may Allaah exalt their mention, will invalidate all excuses that the disbelievers may use on the Day of Resurrection.
Allaah says to His Messenger (what means): "Your duty is to convey (the Message)." [Quran 42:48]. And (what means): "The Messenger's duty is but to convey the Message." [Quran 5:99] This applies to the Message of the Prophet to the disbelievers. He only had to deliver the Message that Allaah sent him with, explain to them their obligations to Allaah and warn them of eternal torment if they insisted on rebellion and disbelief.
Allaah says, (what means): "Say: Obey Allaah and obey the Messenger, but if you turn away, he is only responsible for the duty placed on him (to convey the Message) and you for that placed on you. If you obey him, you shall be on the right guidance. The Messenger's duty is only to convey (the Message) in a clear way." [Quran 24:54]
This Ayah (verse) explains the goals of the Message, the essence of what it contains and the role of the Messenger, sallallaahu ’alayhi wa sallam, and the community he was sent to. The role of the Messenger was to deliver the Message, and the role of those receiving the Message was to hurry to the obedience of Allaah and His Messenger ; if they refused then Allaah's punishment will be justly theirs.
Allaah Says, (what means): "Say (O, Muhammad to the disbelievers): My Lord pays attention to you only because of your invocation to Him. But now you have indeed denied (Him). So the torment will be yours forever." [Quran 25:77] This means that the punishment of Allaah will certainly be felt by those who reject the Message of the Prophets, may Allaah exalt their mention, after being aware of them.
C - Establishing a Muslim Nation:
Allaah Says, (what means): "He it is Who sent among the unlettered ones a Messenger (Muhammad) from among themselves, purifying them, and teaching them the Book (this Quran), and Al-Hikmah (the Sunnah). And verily, they had been before in manifest error." [Quran 62:2]
The Message of Prophet Muhammad was a fulfillment of the prayers of Ibraaheem (Abraham) and Ismaa'eel (may Allaah exalt their mention. They said, while raising the building of the Kaa'bah in Makkah (what means): "Our Lord! Send amongst them a Messenger of their own, who shall recite unto them Your verses and instruct them in the Book (the Quran) and Al-Hikmah, and sanctify them. Verily! You are the All-Mighty, the All-Wise." [Quran 2:129]
Allaah accepted their prayer and sent this honorable Prophet Muhammad and established for him the best nation ever, as Allaah Himself proclaimed (what means): "You (true believers in Islam) are the best of peoples ever raised up for mankind, you enjoin righteousness and forbid evil, and you believe in Allaah." [Quran 3:110]
Allaah praised the Prophet's companions in many verses in the Quran, (which mean) : "Muhammad is the Messenger of Allaah, and those who are with him are severe against disbelievers, and merciful among themselves. You see them bowing and falling down prostrate (in prayer), seeking Bounty from Allaah and (His) Good Pleasure. The mark of them is on their faces (foreheads) from the traces of (their) prostration (during prayers). This is their description in the Torah. But their description in the Gospel is like a (sown) seed which sends forth its shoot, then makes it strong, it then becomes thick, and it stands straight on its stem, delighting the sowers - that He may enrage the disbelievers with them. Allaah has promised those among them who believe and do righteous good deeds, forgiveness and a mighty reward." [Quran 48:29]
D - Making Islam (The Message Of Allaah) Dominant:
The fourth reason for sending Prophet Muhammad is to establish Islam as the dominant religion above all, till the Day of Judgment. Allaah Says, (what means): "He it is Who has sent His Messenger (Muhammad) with Guidance and the Religion of Truth (Islam), that He may establish (make) it (Islam) superior over all religions. And All-Sufficient is Allaah as a Witness." [Quran 48:28]
And also (what means): "It is He Who has sent His Messenger with Guidance and the Religion of Truth to establish (make) it superior over all religions even though the Polytheists hate (it)." [Quran 9:33]
To establish it means to make its domination clear above all religions. Allaah made this promise a reality whenever the Muslims sought victory, equipped with the right belief. A look at the past will show that Muslims conquered all the powers and religions of the world by using their reasoning first, and then using the sword.