Then Allaah did not leave them having to rely upon the intellect and inclination to the correct way which He gave them, rather He also sent Messengers to them as bringers of good tidings and warners. He sent the Books along with them so that they would be a reference for them in those matters where they disagreed, so that no excuse would remain for the people and the proof would be established against them. Then after the sending of the Messengers there would be no remaining excuse for them before Allaah.
Then He obligated upon all the nations obedience to those chosen and excellent Messengers, and that they should follow them and comply with their way. Then 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, may Allaah's praises and blessings of peace be upon them, brought to their 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 except that they warned the people against it.
From 'Abdullaah ibn 'Amr ibn al-'Aas, (رضي الله عنه), who said,
"We were on a journey and stopped to alight at a place. Some of us set up tents, others competed with one another in shooting arrows, and others grazed the animals. Then an announcer for Allaah's Messenger (صلى الله علیه وسلم) announced that we should gather for the Prayer. So we gathered around Allaah's Messenger (صلى الله علیه وسلم) and he 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 and warn them against every evil that he knew. Then as for this nation of yours, its security and well-being is at its beginning and it will be struck in its later part by afflictions, things which you would find disagreeable, and trials will come in succession each one making the previous one seem trifling. So the Believer will say, "This will cause my destruction," then it will be removed, and a further trial will come and the Believer will say, "Rather this is the one." So whoever wishes to be saved from the Fire and to enter Paradise should meet his end whilst having true eemaan in Allaah and the Last Day, and let him treat the people as he himself loves to be treated. Then whoever gives pledge of allegiance to a ruler and gives him his hand and heart sincerely, then let him obey him as far as he is able. Then if another comes to dispute his authority, then strike the neck of the later one.'" 
This was the Messengership 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 what did they concentrate upon? There are a number of basic principles upon which their calls were based, and which were the starting point for calling the people to Allaah. These fundamental points and principles are:
- The Hereafter. 
These three principles are the point of convergence of their calls, and are their fundamental principles. These are given the greatest importance in the Qur'aan and are fully explained in it. They are also its most important goals upon which it centres and which it continually mentions. It further quotes intellectual and physical proofs for them in all the Soorahs and in most of its stories and examples. This is known to those who have full understanding, are able to consider carefully and comprehend well. All the Books revealed by Allaah have given great importance to these points and all of the revealed ways are agreed upon them. Then the most important and sublime of these three principles, and the most fundamental of them all is tawheed of Allaah, the Blessed and the Most High. This is to be found in most of the Soorahs of the Qur'aan, with its three well known categories, indeed it is found actually in every Soorah of the Qur'aan. This is because the Qur'aan comprises:
- Information about Allaah and His names and attributes, and this is, the tawheed of Knowledge (al-Tawheedul-'Ilmee al-Khabree).
- A call to worship Him alone and to attribute no other as a partner to Him in worship, and to reject everything else to which worship is directed besides Him. So this is tawheed of intention and action (at-Tawheedul-Iraadee at-Talabee).
- Orders and prohibitions and a command to be obedient to Him, and this is from the rights of tawheed and is a completion of it.
- Information concerning the honour which He gives to the people of tawheed and the favours which He bestows upon them in this world and the Hereafter. So this is the reward for tawheed.
- Information concerning shirk, and the punishment which He sends upon its people in this world, and the severe torment they receive in the Hereafter. So this is the punishment for those who abandon tawheed.
So all of the Qur'aan is about tawheed and its rights, and its reward; and about the seriousness of shirk, its people and the punishment they receive for it. 
 Reported by Muslim (Eng. trans. 3/1025 no.4546), an-Nasaa'ee and Ibn Maajah.
 Imaam ash-Shawkaanee wrote a book in explanation of these three principles, entitled, Irshaadul-Fuhool ilattifaaqish-Sharaai' 'alat-Tawheed wal-Ma'aad wan-Nubuwwaat', in it he quotes proofs from the Qur'aan and the Tawraat and Injeel.
 Sharh at-Tahaawiyyah, p.88, 1st Edition. 1392 al-Maktabul-Islaamee, and the basis if this is taken from the words of Ibn Taymiyyah and his student Ibnul-Qayyim, rahimahumullaah, and occurs in Madaarijus-Saalikeen of Ibnul-Qayyim (3/450).