Mankind was a single nation

From that which has been established in the Shariah (prescribed law) is that mankind was, in the beginning, a single nation following true Tawheed, then gradually they got involved in shirk (associating partners with Allah).
Allah the Most Blessed, the most High says: "Mankind was one Ummah, then Allah sent prophets bringing good news and warnings." (Qur'an, 2:213)
Ibn Abbas (may Allah bless him) said: "Between Noah and Adam were 10 generations, all of them followed Shariah, then they differed. So Allah sent prophets as bringers of good news and as warners."
Ibn Urwah Al-Hanbali said: "This saying refutes those historians from the People of the Book who claim that Qabil (Cain) and his sons were fire worshippers."
In it is also a refutation of some of the philosophers and atheists who claim that the (natural) basis of man is shirk, and that Tawheed evolved in man! The preceding verse falsifies this claim, as do the two following authentic Hadith: The Prophet's (peace be upon him) said in Hadith Qudsi that Allah said: "I created all my servants upon the true religion (upon Tawheed, free from shirk). Then the devils came to them and led them astray from their true religion. They made unlawful to people that which I had made lawful for them, and they commanded them to associate in worship with Me, that for which I had sent down no authority."
The Prophet (peace be upon him) also said: "Every child is born upon the fitrah (pure human nature) but his parents make him a Jew or a Christian or a Magian. It is like the way an animal gives birth to a natural offspring. Have you noticed any born mutilated, before you mutilate them?"
Abu Hurayrah said, "recite if you wish: 'So direct your face toward the religion, inclining to truth. [Adhere to] the fitrah of Allah upon which He has created [all] people. No change should there be in the creation of Allah . That is the correct religion, but most of the people do not know.'" (Qur'an, 30:30)
After this clear explanation, it is of the upmost importance for the Muslim to know how shirk spread amongst the believers, after they were muwahhidin (people upon Tawheed). Concerning the sayings of Allah the Most Perfect about the people of Noah: "And they have said, 'You shall not forsake your gods, nor shall you forsake Wadd, nor Suwaa, nor Yaghuth, nor Yauq, nor Nasr." (Qur'an, 71:23) It has been related by a group from the salaf (pious predecessors), in many narrations, that these five deities were righteous worshippers. However, when they died, Shaytan (Satan) whispered into their people to retreat and sit at their graves. Then Shaytan whispered to those who came after them that they should take them as idols, beautifying to them the idea that you will be reminded of them and thereby follow them in righteous conduct. Then Shaytan suggested to the third generation that they should worship these idols besides Allah the Most High and he whispered to them that this is what their forefathers used to do!
So Allah sent to them Prophet Noah (peace be upon him), who commanded them to worship Allah alone. However none responded to his call except a few. Allah Almighty related this whole incident in Qur'an (Surah Nuh). Ibn Abbas relates: "Indeed these five names of righteous men from the people of Nuh. When they died Shaytan whispered to their people to make statues of them and to place these statues in their places of gathering as a reminder of them, so they did this. However, none from among them worshipped these statues, until when they died and the purpose of the statues was forgotten. Then (the next generation) began to worship them."
The likes of this has also been related by Ibn Jarir At-Tabari and others, from a number of the salaf (pious predecessors). In Ad-Durral-Manthoor (6/269): Abdullah ibn Humaid relates from Abu Muttahar, who said: Yazid ibn Al-Muhallab was mentioned to Abu Jafar Al-Baqir (d.11H), so he said: "He was killed at the place where another besides Allah was first worshipped. Then he mentioned Wadd and said: "Wadd was a Muslim who was loved by his people. When he died, the people began to gather around his grave in the land of Babil (Babel), lamenting and mourning. So when Iblees (Satan) saw them mourning and lamenting over him, he took the form of a man and came to them, saying: "I see that you are mourning and lamenting over him. So why don't you make a picture of him (i.e. a statue) and place it in your places of gatherings so that you may be reminded of him." So they said: "Yes, and they made a picture of him and put in their place of gathering; which reminded them of him. When Iblees saw how they were (excessively) remembering him, he said: "Why doesn't every man among you make a similar picture to keep in your own houses, so that you can be (constantly) reminded of him." So they all said "yes." So each household made a picture of him, which they adored and venerated and which constantly reminded them of him. Abu Jafar said: "Those from the later generation saw what the (previous generation) had done and considered that ... to the extent that they took him as an ilah (deity) to be worshipped besides Allah. He then said: "This was the first idol worshipped other than Allah, and they called this idol Wadd."
Thus the wisdom of Allah the Blessed, the Most High was fulfilled, when he sent Muhammad (peace be upon him) as the final Prophet and made his Shariah the completion of all divinely prescribed laws, in that He prohibited all means and avenues by which people may fall into shirk, which is the greatest of sins. For this reason, building shrines over graves and intending to specifically travel to them, taking them as places of festivity and gathering and swearing an oath by the inmate of a grave; have all been prohibited. All of these lead to excessiveness and lead to the worship of others than Allah the Most High.
This being the case even more so in an age in which knowledge is diminishing, ignorance is increasing, there are few sincere advisers (to the truth) and Satan is misguiding mankind and to take them away from the worship of Allah the Blessed, the Most High alone.