General Information:

This is the next part of John's vision. He describes suddenly seeing thrones and the souls of believers.

who had been given authority to judge

This can be stated in active form. AT: "whom God had given authority to judge" (See: rc://en/ta/man/translate/figs-activepassive)

This can be stated in active form. AT: "whose heads others had cut off" (See: rc://en/ta/man/translate/figs-activepassive)

"because they had spoken the truth about Jesus and about the word of God"

"They came back to life" or "They became alive again"