Usually an unbeliever is someone who has not yet met Jesus. OR ... he is someone who has been presented the wrong image of Jesus. There are many who simply do not understand Jesus. They have not rejected Him ... for they have not been presented the correct view. Satan is out there working hard to present the wrong image. I don't think that God will condemn someone for rejecting the wrong Jesus.
We sometrimes refer to a person who is not a ASeventh-day Adventists as an unbeliever, but the unbeliever refered to in the Bible was a Jew and another person who did not believe in Jesus as the Savior. Hence, those who believed and accepted were believers and those who did not were unbeliever.