The people of the book were in consensus that Allah took the covenant of the Prophets, [saying], "Whatever I give you of the Scripture and wisdom and then there comes to you a messenger confirming what is with you, you [must] believe in him and support him." (3:81) and that they asked to believe and follow this covenant! You may read traces of that if you check for example the Deuteronomy 18:15-19 (21st Century king James translation), John 14 26 (NIV). So they knew that a Prophet may come as they had this written in their books and Muhammad () therefore was a clear evidence. But they refuted him, because they were expecting a Messenger out of Bani Israel as many other translations of the above quotes indicate.
Allah also say that afterwards they've got divided, some of them believed and followed him, some believed Muhammad () but didn't follow him etc.
This is one part the disbelievers or pagans have tried to challenge Muhammad by asking him Questions etc. (in order to check whether he's a Prophet or not) and these were answered by him and he also came with a book or revelation: the Quran.