By His wisdom, Allah, may He be exalted, never sent any Messenger except He gave him signs to prove his truthfulness and the soundness of his message. Each of the previous Prophets was sent only to his own people, and his call was not for all of mankind and was not to abide until the Day of Resurrection. Therefore Allah gave them tangible signs, and whoever saw them would believe in them, then whoever heard of these signs among the generations who came soon afterwards, would also believe in them. Then when the reports of them faded away, Allah sent another Prophet with other signs, and so on, such as the signs of Moosa and ‘Eesa (peace be upon them).
But the case of our Prophet Muhammad (blessings and peace of Allah be upon him) is different from that of all the other Prophets, for He is the Seal of the Prophets, and there is no prophet after him; his call is for all of the jinn and mankind, and is to abide until the Day of Resurrection. So it was appropriate that his greatest sign, that proves his sincerity and truthfulness, should abide until the Day of Resurrection, preserved by Allah and protected from change and alteration, to establish proof against the stubborn and be believed in by those of sound hearts. So the greatest sign of the Prophet (blessings and peace of Allah be upon him) is the Holy Qur’an, along with what Allah also gave him of visible signs, such as the splitting of the moon and so on.
Concerning that the Prophet (blessings and peace of Allah be upon him) said: “There is no Prophet but he was given signs because of which people believed in him. That which I have been given is revelation that Allah has revealed to me, and I hope that I will have the greatest number of followers on the Day of Resurrection.” Narrated by al-Bukhaari (7274).
What this means is that the Holy Qur’an is the greatest of the signs given to the Messenger (blessings and peace of Allah be upon him). It is a miracle in the utmost sense of the word; in other words, it is beyond people, and is in fact beyond the jinn and mankind, to produce anything like it. Allah, may He be exalted, says (interpretation of the meaning):
“Say: ‘If mankind and the jinn were together to produce the like of this Qur’an, they could not produce the like thereof, even if they helped one another”
Allah, may He be exalted, challenged them to produce the like thereof, or to produce just ten soorahs like its soorahs, or to produce a single soorah only, like its soorahs, even if it was like the shortest soorah that does not take up more than a single line, such as Soorat al-Kawthar or Soorat al-Ikhlaas, and the like.
Allah, may He be exalted, says (interpretation of the meaning):
“Or they say, ‘He (Prophet Muhammad (blessings and peace of Allah be upon him)) forged it (the Qur'an).’ Say: ‘Bring you then ten forged Soorahs (chapters) like unto it, and call whomsoever you can, other than Allah (to your help), if you speak the truth!’”
“And if you (Arab pagans, Jews, and Christians) are in doubt concerning that which We have sent down (i.e. the Qur’an) to Our slave (Muhammad (blessings and peace of Allah be upon him)), then produce a Soorah (chapter) of the like thereof and call your witnesses (supporters and helpers) besides Allah, if you are truthful”
Allah, may He be exalted, has truthfully told us that they will never be able to do any such thing, as He says (interpretation of the meaning):
“But if you do it not, and you can never do it, then fear the Fire (Hell) whose fuel is men and stones, prepared for the disbelievers”
For more than fourteen hundred years, the enemies of the Qur’an have been trying to meet this challenge, which is still valid and still stands, and the Qur’an keeps telling them that their religions are false and that they are regarded as foolish, and it warns them of the Fire. This means that they have had a motive to produce something like it – namely the motive to prove the Qur’an to be false and their desire to show it to be false before people.
There is nothing to stop them from doing that, and there is nothing to prevent them from speaking or writing or responding to this challenge.
Yet despite all that, we have never heard of anyone among them – and they number in the billions, and include eloquent and well-spoken people, literati, writers, scholars and the like…
But we have never heard of anyone among them who accepted this challenge and was able to produce even a single line like the shortest soorah of the Holy Qur’an… Apart from a few miserable attempts made by some of the eloquent Arabs, which caused them to become a laughing stock among their own people before they became a laughing stock among the Muslims!
This challenge still stands until today, and they keep hearing it and will continue to hear it until the onset of the Hour.
The jinn and mankind are incapable of producing anything like it.
After all that, how can it be right to say that it is possible for any person to produce three lines written in an elegant style that matches the eloquence of the Qur’an?!
This possibility is merely hypothetical, as it has no existence in reality, because if that were possible for them, why have they not done it?
So the challenge still stands:
“Let them then produce a recital like unto it (the Qur'ân) if they are truthful”
“then produce a Soorah (chapter) of the like thereof”
(Source - Islam Q&A)