What is the necessary knowledge of the religion that needs to be absolutely learned?

The necessary knowledge that needs to be learned is the 6 fundamentals of belief included in Amantu, the obligatory worshipping like daily prayers, fasting, hajj (pilgrimage), zakat (almsgiving) and the actions like killing a person, having alcohol, and fornicating that are regarded haram by the religion. (32 fard and 54 fard)

It is necessary for every Muslim to know them in detail and to believe them.