Answer: People who do not know God and do not obey the Gospel of our Lord Jesus will begin their punishment on the day the Lord Jesus comes to be glorified in His holy people and to be marveled at among all those who have believed, (verses 8 and 10). In other words, their punishment begins on the Judgment Day.
MORE INFORMATION AND/OR OTHER SCRIPTURE REFERENCES:
2 Peter 3:7: "By the same Word the present heavens and earth are reserved for fire, being kept for the day of judgment and destruction of ungodly men."
