Tafsir of Qaf 50:15

"Were We exhausted by the first creation?" To be ‘ayya (exhausted/incapable) regarding a matter means one does not know the way to perform it. The interrogative hamza is for denial. The meaning is: We were not incapable—as they know—of the first creation, so that We would be incapable of the second.

Furthermore, they do not deny Our power over the first creation; their admission of that inherently includes an admission of the power to resurrect.

"Nay, they are in confusion regarding a new creation." Meaning: in a state of mixing and doubt. Satan has confused them and bewildered them. From this is the saying of Ali (may Allah be pleased with him): "O Harith, it is confused for you; know the truth, and you will know its people."

Satan’s confusion of them is his whispering to them that the resurrection of the dead is something outside of the norm. Because of this, they abandoned the correct analogy: that He who is capable of originating is even more capable of repeating.

If you ask: Why was "new creation" (khalq jadid) used with the indefinite article, and why was the "first creation" not defined? I say: The intent behind using the indefinite is to signify a new creation of great significance and intense state. It is the duty of one who hears of it to be concerned, to fear, and to investigate it, rather than remaining in confusion regarding such a matter.

"And We have certainly created man and We know what his soul whispers to him, and We are closer to him than [his] jugular vein."