Who was the first person in the bible to die for believing in God?
The first person in the Bible to die for believing in Jesus as the Messiah, the Son of God, was Stephen (Acts 7). There were a number of people in the Old Testament who died for their belief in God. Most of them were prophets mentioned in 2 Kings or 2 Chronicles.
If I were to pick someone, though, as the first in the Bible to die for his belief in God it would have to be Abel, in Genesis 4. He made an offering to God that was pleasing. His brother made an offering that wasn't accepted. Cain got angry, and rather than blaming himself for not doing right he chose to kill his brother. Thus Abel is not only the first in the Bible to die, he is the first murder victim and the first to die as a direct result of his belief in and obedience to God.