The Quran explains this by saying that those born in later times were all initially born at the time of the creation of Adam and, at that time, God had directly addressed all these human souls.
This event is thus alluded to in the Quran:
‘[Prophet], when your Lord brought forth the offspring from the loins of the Children of Adam and made them bear witness about themselves, He said, ‘Am I not your Lord?’ and they replied, ‘Yes, we bear witness that You are.’ So you cannot say on the Day of Resurrection, ‘We were not aware of this’. ( 7:172 ).
The Quran, for man, is in essence already known to him, rather than an entirely unknown entity. In reality, the Quran is the unfolding of the human mind.
The Quran is a preserved divine guidance. It addresses the nature of man. It is the answer to a man’s all queries. It tells a man the secret of success in both the worlds, the present as well as the next world. It is not only a book of ideology rather it has narrations from history which attest to its conquering power.
Professor Phillip Hitti expressed it in his book ‘The History of the Arabs’ in the following words:
‘The religion of Muslims has conquered, where their arms had failed.’
So, those who arise with an intention to do dawah work need not any other thing. The Quran is enough to fulfil their all missionary requirements.