Is a person considered a Muslim merely by uttering the Two Testimonies, or is it a prerequisite for being a Muslim to learn matters of Tawhid (monotheism)?

The madhhab (school of thought) of Ahl al-Sunnah wa al-Jama'ah is that whoever utters the two Shahadas (declarations of faith) with certainty, ready to act upon their requirements, is a Muslim. It is not a condition for the validity of his Islam that he learn matters of creed. This is indicated by the hadith: "I have been commanded to fight the people until they testify that there is no god but Allah and that Muhammad is the Messenger of Allah," and the hadith of the slave girl whom the Prophet, peace and blessings be upon him, asked: "Where is Allah?" She said: "In the heaven." He asked: "Who am I?" She said: "You are the Messenger of Allah." He then said: "Emancipate her, for she is a believer."