It is the first pillar of Islam.

One of the reasons for the fast growth of the Muslim community has been its openness to new members. Children born to Muslim parents are automatically considered Muslim. At any time, a non-Muslim can convert to Islam by declaring himself or herself to be a Muslim. A person’s declaration of faith is sufficient evidence of conversion to Islam and need not be confirmed by others or by religious authorities. Islam is the fastest growing religion in the world.

The absolute focus of Islamic piety and faith is Allah, the supreme, all knowing, all-powerful, and above all, all-merciful God. The Arabic word Allah means “the God” and this God is understood to be the God who brought the world into being and sustains it. By obeying God's commands, human beings express their recognition of and gratitude for the wisdom of creation, and live in harmony with the universe.

The first pillar or base of being a Muslim is to declare the faith to Islam ( this pillar is called Al-Shahada).To enter Islam the non-Muslim only need to declare his faith by saying " Ashad ana la ilah illa Allah wa ana Muhammad rasul Allah" which in Arabic means that "I bear witness that there is no god but the one God (Allah) and that Muhammad is his prophet."

Just by saying so the person considered as a Muslim and he becomes a member in the Muslim community.