There are two basic and fundamental conditions to be a Muslim or Muhammadan;
1- Oneness of Allah Almighty.
2- Prophet Muhammad (PBUH) is the last prophet of Allah.
Simply stated any person who professes the Muhammadan religion, that is, acknowledges, that there is but one God, and that Muhammad PBUH is his Prophet, is a Muhammadan. Such a person may be a Muhammadan by birth or he may be a Muhammadan by conversion. It is not necessary that he should observe any particular rites or ceremonies, or be an orthodox believer in that religion; no court can test the gauge the sincerity of religious belief. It is sufficient if the professes the Muhammadan religion in the sense that he accepts the unity of God and the Prophetic character of Muhammad PBUH.
For becoming a Muslim all authoritative books of Islam are agreed that if a person believes in the unity of God(Allah) and Muhammad PBUH to be his prophet and also says that he is Muslim then he becomes a Muslim and no other formalities or rituals are to be gone through by him. There is unanimity among all Islamic scholars of all thoughts that ‘Kalma Tayyaba’ means ‘thee is no god except Allah and Muhammad PBUH is his messenger’. To be a Muslim a person must believe on ‘Kalma Tayyaba’. A person born a Muslim remains Muslim until he renounces the Islamic religion.