With all due respect, a person like that does not believe in Islam because he is not keeping an open mind. Simple. There is no concrete evidence pointing to the lack of a God. We believe that if somebody genuinely wants to learn about Islam and its teachings, he will in all likelihood accept Islam, as was the case with many, many people, back then when Islam was being revealed and even nowadays. If not, who knows? Allah might forgive that person because he tried his best to learn about Islam.
I respectively disagree. I do know some people that have researched on Islam but have come to the conclusion that it isn't god's religion. There are also those who leave Islam after so many years as Muslims. Of course, we don't know if they were strong Muslims but it just shows that Muslims can't all be grounded to their faith.