Allah, the all Wise, all Knowledgeable, all Merciful and all Just created this Universe in order to test and examine His obedient slaves and reward them, and to punish the disobedient. He says: “We did not create the heavens and the earth and all that is between them in play. We did not create them except for a true purpose (to test and examine who are obedient, and to reward them, and who are disobedient and punish them), but most of them do not know.” [Al Quran Chapter adh-Dhukkhan (44): 38-39]
Allah also made clear the main goal for the creation of mankind saying: “I did not create Jinn and Mankind except for My worship” [Al Quran Chapter adh-Dharia (56): 56]
In addition to His Great Blessings and Great eminence to mankind, Allah, the all-Merciful also gave them the blessing of intellect, which raises them to the level of taking the responsibilities given by Allah. And it enables them to achieve and comprehend His responsibilities. He also provided them with the natural inclination to the truth (Fitra)
“So set your faces upon the true religion, firmly upon His religion and obedience to Him, the true religion of Islamic Monotheism (al-Fitr) upon which He created mankind. There is to be no change to the religion of Allah. That is the true religion, yet most people do not know.” [Al Quran Chapter Room (30): 30]
Iyaad Ibn Himaar al-Mujaashi’ee (radhi allahu anhu) said that the Prophet (sallallahu alaihi wa-sallam) gave a khutbah, “Indeed, my Lord, the Mighty and Majestic, ordered me to teach you that which you do not know. From that which he taught me this day: “This wealth which I confer upon My servants is lawful for them, and I created my servants upon the true religion, but the devil came to them and turned them away from their true religion, and forbade for them that which I made lawful for them, and ordered them to associate in worship with Me that for which I sent down no authority.” [Muslim eng. trans. 4/1488 no. 6853] 
BUT, Allah, the Exalted, did not leave His slaves to rely upon their intellect and inclination to the correct way, rather He also sent the Messengers to them as bringers of good tidings and warners. He sent the Book along with them so that they would be a reference for them in those matters they disagreed, and that no excuse would remain for the people and the proof would be establishes against them. Then after sending of the Messenger there would be no remaining excuse for them before Allah.
Then He obligated upon all the nations the obedience to those chosen and excellent Messengers, and that they should follow them and comply with their way. He sent down the severest punishment upon those who belied and rejected them in this world, and He will send worse and more terrible punishment upon them, eternal, never-ending torment in the place of just recompense (i.e. Hell). So what was the message which these noble, chosen men (alaihi as-salaam), bring to the people?
Indeed, their mission covered everything good and banished every evil. They brought to mankind everything needed for their well being and happiness in this world and the Hereafter. Indeed, there is nothing good except that they informed the people of it and nothing evil that they warned the people against it.
Abdullah ibn Amr ibn al-Aas (radhi allahu anhu) said: “… Prophet (sallallahu alahi wa-sallam) said: ‘There was never a Prophet before me except that it was a duty upon him that he should guide his nation to every good that he knew an warn them against every evil that he knew…” [Muslim eng. trans. 3/1025 no. 4546]
This was the Messenger-ship of the Messengers that they should guide to every good and warn against every evil. However, where did they start, what did they begin with and on what did they concentrate?
Indeed, calling to Allah was the mission of the Messengers, in order to bring the people out from darkness and into light, there are a number of basic principles upon which their calls were based, and which were the starting point for calling to Allah. These fundamentals and principles are:
3. The Hereafter 
These points summarize the call of the Messengers. All the books revealed by Allah have given great importance to these point and all of the revealed ways are agreed upon them. The most important and sublime of these three principles and the most fundamental of them all is Tawheed of Allah, the Blessed and the Most High.
Allah says: “We sent a Messenger to every nation ordering them that they should worship Allah Alone, obey Him and make their worship purely for Him, an that they should avoid everything worshipped except Allah.” [Al Quran Chapter an-Nahl (16): 36]
And the Most High says: “We did not send any Messenger before you, O Muhammad (sallallahu alahi wa-sallam) except that We revealed to him that none has the right to be worshipped except Allah, so make all of your worship purely for Him.’” [Al Quran Chapter al-Ambiya (21): 252]
He, the Most High says after mentioning a number of stories of the Prophets (alaihi as-salaam): “This religion of yours is a single religion and I am your Lord, so worship Me.” [Al Quran Chapter al-Ambiyah (21): 92]
Allah has informed us about some of the individuals from the Prophets, and how they faced their people. We see that all of them proceeded upon the universal way laid down by Allah and they followed the methodology established for all of them by Allah. Not a single one of them is at variance with it, Allah says: “And We sent Nooh to his people saying, ‘I am a clear warner to you, that you should worship none besides Allah. Indeed, if you do not make all your worship purely for Him I fear for you the torment of a painful Day.” [Al Quran Chapter Hood (11): 25-27]
And the Most High says: “And to ‘Aad We sent Hood, saying, ‘O my people worship Allah, making all your worship purely for him, since there is none besides Him having the right to be worshipped by you. Will you not fear your Lord?” [Al Quran Chapter A’araf (7): 65]
The noblest of all Prophets and the last of them; Muhammad ibn Abdullah (sallallahu alahi wa-sallam), the one sent with the greatest, most complete and most comprehensive message, began with what all the Prophets (alaihi as-salaam) began with and started where they started in their dawah; calling to the ‘aqeedah of tawheed, and calling for all worship to be made purely and sincerely for Allah Alone. Allah’s Messenger (sallallahu alahi wa-sallam) started with the principle ‘Witness that none has the right to be worshipped except Allah’ So, the proud and haughty said: “The unbelievers said: ‘Has he made all that is to be worshipped a single God, who alone is to called and hears all invocation. This is a very curious thing which we have not heard of.’ So the leaders amongst them went off saying to one another. ‘Continue in that you are upon and persist in worshipping your idols. He only says this to gain ascendancy over us.” [Al Quran Chapter Saad (38): 5-6]
He, the Exalted also commanded him to call all the people to fulfill and implement this principle and to proceed upon it: “O Mankind, single out your Lord with worship; He who created you and all those who came before you so that you may be of those who seek to avoid Allah’s punishment and anger; those whom Allah is pleased with.” [Al Quran Chapter al-Baqarah (2): 21]
Allah the Most High says: “Say, O Muhammad to all of the people, ‘I am the Messenger of Allah to you all. Allah, to Whom belongs the dominion of the heavens and the earth and everything therein. None has the right to be worshipped but Him. He Who alone gives life and death. So affirm true faith in Allah and in His sole right to worship and believe in His Messenger….” [Al Quran Chapter al-A’araf (7): 158]
The aayat in this regard are many, what we quote here is just an example. With regard to the Sunnah, there are many things clearly showing that Allah’s Messenger started his dawah with Tawheed, and completed it with it:
1. Amr ibn Abasah as- Sulamee (radhi allahu anhu) said: “During the times of ignorance … I heard of a man in Makkha who received revelation. So I set upon my riding beast and came to him. At that time he was in hiding, due to the oppression of his people. So I behaved in a way that enabled me to gain access to him in Makkah. So I said to him, “What are you?” he replied “I am a Prophet.’ So I said: “And what is a Prophet?’ He said: ‘Allah has sent me as a Messenger.’ So I said: ‘and what is it that He has sent you with?’ He said: ‘I have been sent to order the joining of ties of relationship, to break the idols, so that Allah is worshipped Alone and nothing at all is associated in worship along with Him” ….” [Muslim eng. trans. 2/395 no. 1812 and Ahmad in al-Musnad 4/112]
2. Also from the questions which Hirqal (Heraclius) asked Aboo Sufyan, at the time of the peace of Hudaybiyyah, about Allah’s Messenger (sallallahu alahi wa-sallam) was that he asked Aboo Sufyan. “What does he (Muhammad (sallallahu alahi wa-sallam) command you?” So Aboo Sufyan replied: “He says, ‘Worship Allah Alone and do not worship anything else along with Him, and abandon what your fathers say. And he orders Prayers, charity, and the joining of the ties of relationship.’” [Bukharee eng. trans. 1/7/ no. 6]
Consequently, the Prophets (alaihi as-salaam) did not come to bring about the downfall of one state to replace it with another. They did not seek after sovereignty, nor did they organize parties for this reason. Rather, they came for the guidance of mankind and to save them from the misguidance and shirk, and to take mankind out of darkness into light.
Abu Hurairah (radhi allahu anhu) said: “Jibreel sat with Messenger (sallallahu alahi wa-sallam) and looked to the heavens and saw an Angel descending, so Jibreel said: ‘This Angel has never descended since he was created until now.’ So when he descended he said: ‘O Muhammad (sallallahu alahi wa-sallam)! Your Lord has sent me to you, saying: ‘Shall He make you a Prophet-king or a Slave-Messenger?” Jibreel said: “Show humbleness to you Lord, O Muhammad.’ He said: ‘Rather a Slave-Messenger.’”
So, in what has preceded, we come to know the methodology of the Prophets, their priority in calling to Allah being ‘the aqeedah of tawheed’, witnesses the priority of Faith in the true religion of Islaam and its importance. It proves that the actions are to be preceded by strong faith and that without it none of the actions reach the height of acceptance.
Furthermore, the Messengers were sent with glad tidings and warners;
“And We sent not the Messengers save as bearers of good news and warners.” [(18): 56]
In this regard Prophet (sallallahu alahi wa-sallam) gave an example about himself. He said: “The similitude of me and the message with which Allah has sent me, is like a man who came to some people and said: “I have seen the enemy forces with my own eyes and I am a naked warner (to you)  So, save yourselves, save yourselves! A Groups of his people obeyed him and went out at night, slowly and steadily and were safe; while another group belied him and stayed in their place until the morning when the army ran them and destroyed them. Such is the similitude of those who obey me and follow what I have brought (i.e. Message) and of those who disobey me and belie the Truth which I have brought.” [al-Bukharee, Muslim and others]
These glad tidings of the Great Bounties of Allah and warnings of the disgraceful torments brought by the Messengers are key elements that influence the human soul. The soul aspires for what is good and likes to drive away evil. These glad tidings are something which brings sweetness to the heart and satisfaction to the soul as well as pleasure to the innermost senses.
The Message of the Messengers, by the Mercy of the Allah, revives and enlightens the soul of the believer. This enlightenment is the revelation of Allah. His revelation leads mankind from the deepest of darkness, the darkness of Shirk and Ignorance to the Light of Truth and Islaam: “Allah is the Wali (Protection or Guardian) of those who believe. He brings them out of the darkness into the light.” [Al Quran Chapter al-Baqarah (2): 257]
The Messenger and the Message is an Evidence against the Disbelievers:
Allah, Most Honored and Exalted sent His Messenger and His Books so that there can be no excuse for mankind on the Day of Judgement: “Messengers as bearers of good news as well as of warning in order that mankind should have no plea against Allah and His Messenger.” [(4): 165]
“And (Remember) the Day We shall raise from every nation a witness against them from amongst themselves from among themselves. And we shall bring you (O Muhammad (sallallahu alahi wa-sallam) as a witness against these.” [(16): 89]
The Legislative Policy of the Nation: The Message of Allah is a Legislative policy for the nation: “So judge between them by what Allah has revealed” [Al Quran Chapter al-Maidah (5): 48]
As a complete implication, the Message of Allah deals with: The rights of Allah on His Bondsmen, the Rights of man on His Creator, The Oneness of Allah and its influence on the creation, Knowledge, Admonition, The Wisdom Behind the Creation, Righteousness, Interactions, Refutation of Incorrect Beliefs and Deviations, Lawful and unlawful, Migration and Jihad. In totality it is the complete code of a righteous living, which results in delight, pleasure and harmony in this life and the Hereafter.