A warrior in the most simplest of terms, is someone who fights, more specifically fights in a war. Generally, warriors fight for something they believe in with strong conviction, though a few cases throughout history are found of mercenary warriors for hire.
A warrior may fight physically, like an army soldier, a 17th-century American Indian protecting his home, or the famous Japanese Samurai. But a warrior may also fight with words, like an impassioned journalist or blogger, or may fight with spirit.
Christianity has long recognized the importance of being a warrior: "Onward Christian soldiers marching on to war, with the cross of Jesus going on before..." and Warriors of God.