A Muslim is someone who practices Islam; that is, who willingly submits to the revealed directives of God. Everything in the universe is inherently submissive to God, functioning according to the natural laws created by Him. Human beings are physically Muslim in that their bodies function according to the genetic program set by God for the period of time He has decreed.
A Muslim in the religious sense, however, is someone who consciously commits himself to the worship of God alone, not according to his own inclinations or those of other men but according to the method conveyed by God through His appointed prophets. The Qur'an gives many examples of people who lived before Muhammad and believed in and obeyed the prophet that God had sent them. They entered Paradise because of their obedience to God, and that is the meaning of Islam.
All the prophets of God, from Adam to Muhammad, and those who believed and followed each of them during the period of his prophethood were called Muslims. But since Prophet Muhammad was the last of God's messengers to mankind, a Muslim can now only be defined as one who accepts and complies with the final, completed message conveyed by God through him. For rationally, the most recently revised legislation always supersedes and invalidates whatever came before it.