Who Is an 'Unbeliever,' According to the Bible?

Who is God referring to in the Bible when He talks about the unbeliever?

The unbeliever refers to the person who rejects Christ or doesn't believe in Him as his personal Savior.

For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life. For God did not send His Son into the world to condemn the world, but that the world through Him might be saved.

He who believes in Him is not condemned; but he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God.
(John 3:16-18)

And He said to them, "Go into all the world and preach the gospel to every creature. He who believes and is baptized will be saved; but he who does not believe will be condemned."
(Mark 16:15-16)

The Bible teaches that God makes a distinction between people according to what they believe about Jesus.

With the exception of the Jewish people with whom He is still dealing with separately, God divides the world into two categories: believers and unbelievers. The believers have heaven promised while the unbelievers have only hell to look forward to because their sins aren't washed in the blood of Jesus. This is why it is so important for the Christian to propagate the spreading of the Gospel.

So, as much as is in me, I am ready to preach the gospel to you who are in Rome also. For I am not ashamed of the gospel of Christ, for it is the power of God to salvation for everyone who believes, for the Jew first and also for the Greek.
(Rom. 1:15-16)