Because we believe in Quran we have to follow the message of Islam
Here are two quotes
And We have not sent thee (O Muhammad) save as a bringer of good tidings and a warner unto all mankind; but most of mankind know not. 34:28

Call unto the way of thy Lord with wisdom and fair exhortation, and reason with them in the better way. Lo! thy Lord is Best Aware of him who strayeth from His way, and He is best aware of those who go aright (16:124)