Know this first
"Know this as the most important thing." See how you translated this in 2 Peter 1:20.
proceed according to their own desires
Here the word "desires" refers to sinful desires that are opposed to God's will. AT: "live according to their own sinful desires" (See: rc://en/ta/man/translate/figs-explicit)
Where is the promise of his return?
The mockers ask this rhetorical question to emphasize that they do not believe that Jesus will return. The word "promise" refers to the fulfillment of the promise that Jesus will return. AT: "The promise that Jesus would return is not true! He will not return!" (See: rc://en/ta/man/translate/figs-rquestion and rc://en/ta/man/translate/figs-metonymy)
our fathers fell asleep
Here "fathers" refers to ancestors who lived long ago. "Fell asleep" is an idiom that means they died. (See: rc://en/ta/man/translate/figs-idiom)
all things have stayed the same, since the beginning of creation
The mockers exaggerate with the word "all", and they argue that since nothing in the world has ever changed, it cannot be true that Jesus will return. (See: rc://en/ta/man/translate/figs-hyperbole)
since the beginning of creation
This can be translated as a verbal phrase. AT: "since God created the world" (See: rc://en/ta/man/translate/figs-abstractnouns)