My friend, you are pointing to the beginning of second Surah - Al-Baqarah. Indeed it says so but it is actually a challenge. Also, it says in the 3rd verse that it is Guidance for 'Muttaqeen' (Those who have Taqwa - fear of God). The subsequent verses go on to explain what are the attributes or qualities of such people (who would find Guidance in Quran).
The important thing to remember is that Quran is for everyone. Dr. Jeffery Lang, Associate Professor of Mathematics at University of Kansas, was an atheist when he read Quran and he accepted Islam because he was won over by Quran:
“Painters can make the eyes of a portrait appear to be following you from one place to another, but which author can write a scripture that anticipates your daily vicissitudes?… Each night I would formulate questions and objections and somehow discover the answer the next day. It seemed that the author was reading my ideas and writing in the appropriate lines in time for my next reading. I have met myself in its pages…”
See for yourself here:
Islam – My Choice | US Professor Dr. Jeffrey Lang – How i embraced islam?
He has written books about his experiences "Struggling to Surrender" and "Even Angels Ask: A Journey to Islam in America". I've read both of them and his perspective is both challenging, and refreshing as it should be - coming from an intellectual who understands his culture & his former religion as well as Islam and the dynamics of many issues involved that likes of me do not usually think about.
However, it is important to note that Quran is not for only the people who already believe. It is for everyone. Even if someone does not believe,
they still get something out of it.