Is a person a Muslim after declaring the two testimonies of faith? Is a person supposed to learn the issue of Tawheed (Islamic monotheism) before being declared a Muslim?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

The opinion of Ahlus-Sunnah Wal-Jamaa'ah states that if a person firmly professes the Two Testimonies of Faith and is fully ready to act accordingly, this person is considered a Muslim. His embracing Islam does not require him to learn the issues pertaining to its creed. In support of this the Prophet mentioned that Allaah commanded him to fight people until they testified that none is worthy of being worshiped but Allaah and that Muhammad is the Messenger of Allaah; performed their prayers; and paid Zakah (obligatory almsgiving). If they did so they would have protection for their life and property, while being held accountable for their inner secrets by Allaah, unless they committed an act requiring a prescribed corporal punishment, an expiation, or murder in Islam for which they deserve to be killed. [Al-Bukhari and Muslim].

The second evidence is the Hadeeth about the slave girl whom the Prophet said was a believer because she testified that Allaah the Almighty is over the heavens and that Muhammad is the Messenger of Allaah. [Muslim] Imaam An-Nawawi stated that these narrations prove that the person who firmly declares the Two Testimonies of Faith is a Muslim and should not be asked to prove that.

Allaah Knows best.