Question: Does mere pronouncement of Shahaadah (Testimony of Faith) suffice for a person to be rightfully called a Muslim, or does one have to do other things in order for their Islaam and Eemaan (faith) to be complete?
Answer: It is enough for a non-Muslim to pronounce the first pillar of Islaam, that is, to bear witness that Laa ilaaha illaa Allaah (there is no deity but Allaah) and Muhammad is the Messenger of Allaah to be rightfully called a Muslim provided that this testimony is pronounced out of one’s certainty, knowledge, true belief, and sincere work. A person will then be asked to perform Salaah (Prayer) and observe other acts of `Ibaadah (worship). When the Prophet (peace be upon him) sent Mu`aadh (may Allaah be pleased with him) to Yemen, he said: Invite them to bear witness that Laa ilaaha illaa Allaah and Muhammad is the Messenger of Allaah. If they do that, then inform them that Allaah has made it obligatory upon them to offer the Five Obligatory Daily Prayers. If they obey, then inform them that Allaah has made it obligatory upon them to pay Zakaah (obligatory charity) which is taken from the rich among them and distributed among the poor. The Prophet (peace be upon him) did not ask Mu`aadh to command them to offer Salaah until their Tawheed (monotheism) and Eemaan in the Messenger of Allaah (peace be upon him) were firmly established. First of all, they are required to declare the Oneness of Allaah and believe in Muhammad as the Messenger of Allaah. Once they embrace Islaam, the former Kaafir (disbeliever) will hold the same rulings applied to Muslims. Then they will be required to offer Salaah and perform all other religious acts. If they refrain from performing these acts, other rulings will be applicable to them... read more here.