19. And Allah knows what you conceal and what you reveal. 20. Those whom they invoke besides Allah have not created anything, but are themselves created. 21. (They are) dead, not alive; and they do not know when they will be resurrected.) Allah tells us that He knows what is hidden in people's hearts as well as what is apparent. He will reward or punish everyone for their deeds on the Day of Resurrection. If their deeds are good then they will be rewarded, and if their deeds are evil, then they will be punished.
Then Allah tells us that the idols which people call on instead of Him cannot create anything, they are themselves created, as Al-Khalil (Ibrahim) said:
("Do you worship that which you (yourselves) carve While Allah has created you and what you make!'') (37:-96).
((They are) dead, not alive) means, they are inanimate and lifeless, they do not hear, see, or think.
(and they know not when they will be resurrected.) meaning, they do not know when the Hour will come, so how can anyone hope for any benefit or reward from these idols They should hope for it from the One Who knows all things and is the Creator of all things.