What is the obligatory minimum of 'Aqeedah (Islamic Creed) that one must learn, and is it obligatory to ask about matters of 'Aqeedah that fall outside this minimum?

The minimum requirement for a person to be a Muslim is to know the meaning of the two testimonies (Shahadatayn) and affirm them, followed by a general knowledge of the pillars of faith. As for the detailed knowledge, its obligation varies depending on the person's situation, environment, and any doubts they may encounter.

Shaykh al-Islam Ibn Taymiyyah was asked about what is obligatory to believe and know. He replied that, in general, what is obligatory is belief in Allah and His Messenger, and affirming everything that the Messenger brought. As for the details, the accountable person must affirm what has been established to them as having been conveyed and commanded by the Messenger. They are not punished for what has not reached them or what they were unable to know in detail; rather, this falls within their general affirmation. What is obligatory for scholars differs from what is obligatory for common people, and what is obligatory for someone raised in an environment of knowledge differs from others. As for what is known to be true through rational deduction without revelation, one is not punished for not believing in it.

As for what must be known, it is what Allah has commanded, such as the knowledge of Zakat (definition) for those on whom it is obligatory, and the knowledge of Hajj for those performing Hajj.

The desired knowledge is what the Prophet (peace be upon him) taught his Ummah. Every person is encouraged to acquire the knowledge that is most necessary and beneficial for them. This varies according to need. For example, it is more beneficial for the general public to know about obligations and recommended acts.

As for certainty (yaqin), it is the tranquility of the heart and the stability of knowledge within it.