Does one who feels safe from Allah’s deception or despairs of His mercy commit an act of major disbelief that expels him from the faith, and what is the wisdom behind that?

Feeling secure from Allah's devices and despairing of His mercy are among the greatest major sins. Some Hanafi jurists have even considered them types of disbelief (kufr). A Muslim must combine fear of Allah with hope for His mercy and pleasure, as both are obligations that cannot be separated, as stated in the Quranic texts and prophetic hadiths.