According to my knowledge, if you are talking from an Islamic point of view then you need to understand that the Islamic concept says that Adam was a prophet of God and the first man on earth who was following the true religion. Here we need to understand that according to the Islamic concept, all the prophets were sent with the same mission and that was to guide the people towards the worship of one God and teach them the difference between good and bad. Islam believes that all the prophets from Adam to Hazrat Muhammad (PBUH) were carrying forward the same message, it started from Adam, after that there came Moses, Jesus and in the end Prophet Muhammad. All these prophets brought extensions of the same message according to the needs of the day that culminated into Islam. According to the Islamic concept and people were supposed to leave their old practices and modify them according to the teachings of the new prophet but what happened was that some people did not leave their old practices and refused the new teachings. Over time new practices added to these and they transformed into different religions. Adam was called a Muslim because he was a true believer in the teachings of God sent at that time.