I believe that Quran is the book of Allah because:;
1:Arabs were very rightly proud of their Arabic language even they called 'Ajam"(means deaf) to non Arabic. Despite low in every field of life they were very literate and sensitive when it comes to the language. At that time Quran reviled and challenged the whole world to bring or produce a single phrase same as Quran if you are real....but until today this challenge is not fulfilled........
2: As we know that not a single religious book except Quran is safe from change, as its natural....... but Allah in Quran claimed that this book will be safe forever........When I see that if the promise of safety was false , then leave the non muslims every sect in the so called muslims would have added their filthy believes in the Quran..(that's why they invented another type of revelation called Ahadees which is easy for them to say whatever crape they want to say about Allah and the prophet (pbuh) and hens now it was very easy to invent the religion of their choice including claiming of prophecy.....etc etc..