The Bible does not state that dying by crucifixion is a requirement or guarantee of being a Christian. The defining factor of being a Christian is having faith in Jesus Christ as Lord and Savior.
While Jesus was crucified, and the cross is a central symbol of Christianity representing his sacrifice, the method of one's death does not determine their faith or salvation. Many Christians throughout history have died in various ways, including martyrdom, but their faith is what defines them as Christian, not how they died.