At that time, in the city of Yathrib (Medina) there was a large group of Jews. They belong to the Book of Ahlul because they have the Torah. However, the book of law in their hands has undergone several changes. So, it is not intact as time was revealed to Prophet Musa.
Unbelievers of Quraysh sent envoys to meet Jewish priests at Yathrib. The messenger told about the emergence of Muhammad as a prophet and messenger of Allah SWT. The envoy also recounted that Muhammad Saw claimed to receive revelations from Allah SWT.
The Quraysh envoy said to the Jewish priests, "You are the leaders of the Jews. You are the first ahlul. You have knowledge of prophethood that we do not have. Then tell us, what is your perspective on Muhammad?".
A Jewish priest replied, "Try to ask three things to him. Ask about a group of young men who lived in ancient times and have amazing stories. Then, ask him about the mighty man who goes east and west. And finally, you ask him about spirit, what spirit is it? If he can answer the first and second questions, and does not explain (answer) the third question, then he is truly a prophet. "
The Messenger of Quraysh returned to Mecca. Once they arrived, they preached and told the words of the Jewish priest to the leaders of the Quraish infidels. Then they together tested the Prophet Muhammad with these 3 questions.
To answer these three questions Allah Almighty revealed the letter of Al-Kahf. With Al-Kahf's letter, he answered 3 questions correctly.
The first question was answered with a story about Ashabul Kahf, a group of young people who entered the cave because they had escaped from the pursuit of a tyrannical, violent, cruel king. Young people are accompanied by a dog. They slept in a cave for more than 300 years. When they wake up, they find that their country has completely changed. Everyone who knows their story is amazed. The Prophet explained the story of the Ashabul Kafi in detail and clearly.
The second question, he answered with the story of King Dzulqarnaini. A mighty king who subjugated many kings and traveled to the east and west world. Until finally, Dhulqarnain met with the people who asked him to build a wall of separation between them with Ya'juj and Ma'juj, so that they were safe from the crimes of Ya'juj and Ma'juj, until the end of the story.
The last (third) question, which is about the spirit, the Rasullullah Saw answers it with the word of Allah SWT in Surat Al-Isra 'verse 85, which means:
"And they asked you (Muhammad) about the soul. Say, the Spirit is included in the affairs of my Lord and I am not given knowledge except a little".
Thus, the Messenger of Allah has answered these three questions very precisely, because he received direct guidance from Allah SWT.
At once, most people from the Quraish infidels believed that the Prophet Muhammad was truly a prophet who sent Allah, and the teachings of Islam that he brought really came from Allah Almighty.
However, even though they know that, many of them still do not want to believe. Their disbelief is due to their sense of pride, arrogance, and love for the world with such great love.