What are the foundations of the Islamic creed?

Knowing the fundamentals of Islamic creed is among the most important matters that a Muslim must attend to and prioritize over others, as Hafiz al-Hakami said in Sullam al-Wusul: "The first obligation upon servants is knowing the Most Merciful through monotheism, for it is the greatest of all commands, and it is of two types, O one who understands." For further understanding, one can refer to books on creed and fatwas related to its meaning and fundamentals.