Allah tells us that there is none to be worshipped besides Him, the One, the Unique, the Lone, the Self-Sufficient. He tells us that the hearts of the disbelievers deny that and are astonished by that:
("Has he made the gods (all) into One God! Verily, this is a curious thing!") (38:5).
(And when Allah alone is mentioned, the hearts of those who do not believe in the Hereafter are filled with disgust, and when those besides Him are mentioned, behold, they rejoice!) (39:45).
(and they are proud) meaning they are too proud to worship Allah, and their hearts reject the idea of singling Him out, as Allah says:
(Verily! Those who scorn My worship they will surely enter Hell in humiliation!) 40:60 So here, Allah says;
(Certainly), meaning truly,
(Allah knows what they conceal and what they reveal.) meaning He will requite them for that in full.
(Truly, He does not like the proud.)