the heavens and the earth came to exist ... long ago, by God's command
This can be stated in active form. AT: "God established the heavens and the earth ... long ago by his word" (See: rc://en/ta/man/translate/figs-activepassive)
came to exist out of water and through water
This means that God caused the land to come up out of the water, gathering the bodies of water together to make the land appear.
through these things
Here "these things" refers to God's word and water.
the world of that time was destroyed, being flooded with water
This can be stated in active form. AT: "God flooded the world that existed at that time with water and destroyed it" (See: rc://en/ta/man/translate/figs-activepassive)
the heavens and the earth are reserved for fire by that same command
This can be stated in active form. AT: "God, by that same word, has reserved the heavens and the earth for fire" (See: rc://en/ta/man/translate/figs-activepassive)
that same command
Here "command" stands for God, who will give the command: AT "God, who will give a similar command"
They are reserved for the day of judgment
This can be stated in active form and can begin a new sentence. AT: "He is reserving them for the day of judgment" (See: rc://en/ta/man/translate/figs-activepassive)
for the day of judgment and the destruction of the ungodly people
This can be stated with verbal phrases. AT: "for the day when he judges and destroys ungodly people" (See: rc://en/ta/man/translate/figs-abstractnouns)