According to the majority of Shafi Islamic Scholars, there are three categories of Sacred Knowledge.
WHAT IS AN INDIVIDUAL MUSLIM REQUIRED TO KNOW
The first category is the personally obligatory (Fard al-ayn) which is required by each and every sane Muslim that has reached the age of puberty.
What an individual is personally responsible to have knowledge of is of the obligatory acts of worship in which he or she is responsible to perform, including ablution, wudhu, the cleaning which is done before one prays; as well as the prayer itself.
In addition to the purification and the prayer an individual Muslim male or female is also required to have knowledge of the basic tenants of faith. It is sufficient for one to simply say that he or she believes with absolute conviction free of any doubt without learning the evidences of the scholastic theologians. However if one falls into the category of being one without such knowledge, he or she was to refrain making comments, judgments, and or giving opinions about matters of faith beyond their scope of knowledge. Knowledge that may benefit one common people may differ from the knowledge in which benefits another. (1)
AN INDIVIDUAL MUSLIM IS NOT REQUIRED TO HAVE KNOWLEDGE OF THINGS WHICH DO NOT CONCERN HIM OR HER
The general consensus is one is not required to have knowledge of a particular act until that person is obligatory to perform it.
As for obtaining knowledge of marriage, business transactions, etc. one needs to obtain knowledge in such areas before one engages in such acts so that he or she does not fall into sin. For example, there are certain obligations upon a husband or wife in marriage, there are certain agreements that may be placed in a marriage contract. In business transactions there are stipulations that must be orally agreed to, however preferably written in a business contract. In addition one must understand that trade, and taxes is permissible but the charging and paying of interests is forbidden.
If one is going to eat food, or wear clothing he or she is obligatory to know what food and clothing is permissible etc.
MUSLIM CHILDREN HAVE RIGHTS AND THERE FOR OBLIGATED TO OBTAIN CERTAIN KNOWLEDGE
In Islam there are rights among individuals. It is the right of the child and the responsibility of his or her parents to teach the child what is obligatory for him or her by time they reach puberty, and if they do not the parents are committing an act of sin. For example, my children are ages 16, 11, and 10. I am required to insecure that my teen ager has knowledge about sex, fornication, marriage, not dating, prayer, purification, theft, drinking, stealing, slander, lying etc. However for my 11 and ten year old I am not required to teach them about sex and or fornication until they have reached an age in which they may be tempted to indulge in such acts.
A MUSLIM IS OBLIGATED TO ASK ABOUT KNOWLEDGE THAT HE OR SHE IS UNSURE OF
If for some reason a person is in a situation in which they are dealing with a matter in which they lack knowledge, they are required to seek guidance from those that have knowledge, and it is obligatory for those with knowledge to provide them with which they seek.
In reference to what Islamic Scholars describe as Knowledge of The Heart, ie anger, jealousy, envy, pride, etc. each individual is also personally responsible to have knowledge about its definitions, causes, and remedies, so that the believing Muslim male or female will know what he or she should to have a pure heart to the best of their ability. There is an opinion amongst a few scholars who believe that if ones heart is free of such diseases than he or she is not obligated to seek out knowledge about such matters.
WHAT A MUSLIM COMMUNITY (UMMAH) IS REQUIRED TO KNOW
Secondly there is a category of knowledge in which is required communally ( Fard Kifaya) there needs to be individuals within the community that possess such knowledge, but these areas of knowledge are not required by each and every individual.
We can look at it as we look at society as a whole. We need doctors, lawyers, teachers, plumbers, and electricians; however each individual is not required to have knowledge of all these things. It is best that just many within the society possess such knowledge so that the needs of the people are met.
A FEW MUSLIMS IN THE COMMUNITY ARE REQUIRED TO MEMORIZE THE QURAN
Traditionally there are thousands if not millions of the billions of Muslims that are Hafiz of Quran. They are the protectors and maintainers of the verses that were sent down. Often learning and memorizing these verses before attempting secular knowledge at a young age as early as four. We find that many individuals, who are Hafiz of Quran, may not even understand the sacred text, but simply have its verses memorized.
In addition to the verses of Quran, many have also memorized Hadith, or have obtained knowledge of Sacred Law such as this, and so on. For knowledge that is not sacred but secular in category, it is also a communal obligation as mentioned above. Society needs people with knowledge of medicine, and mathematics, and biology, however it is not required for all to have such knowledge individually.
SUPEROGATORY KNOWLEDGE THAT IS BENEFITIAL BUT NOT REQUIRED FOR MUSLIMS TO KNOW
Lastly is the category of recommended knowledge, supererogatory. These include non-obligatory acts of worship, and or elaboration beyond a certain amount of communal knowledge.
It can take a Muslim a life time to obtain the basics of those areas of Sacred Knowledge mentioned above. As tradition states it is not only having an understanding of what one has learned, but putting it into action and being an example for others as well. This is true knowledge.
(1) Reliance of The Traveler, Nuh Ha Mim Keller a4-a5