The juristic characterization of pronouncing the two testimonies to enter Islam
Is uttering the shahada a condition for the validity of Islam or a essential element, and what of one who believes in his heart without uttering it?
The essence of faith is assent of the heart; utterance with the tongue is a condition for applying the worldly rulings of Muslims.
Uttering the two testimonies is a condition for the validity of faith for one able to do so, and entering Islam is not valid without it.
The reality of faith is assent in the heart, and uttering the two testimonies is a condition for the outward application of the rulings of Islam; so one capable of uttering it is not judged a Muslim until he does.
A disbeliever's Islam is valid by testifying that there is no god but Allah and that Muhammad is the Messenger of Allah; but if his disbelief lay in denying an obligation or a prophet, or in claiming that the Message was for the Arabs alone, his Islam is not valid until he affirms what he denied or disavows every religion contrary to Islam, along with the two testimonies.