When Isaiah wrote this passage he wasn't writing to the Gentiles, he was writing to the Jews. Zion in this passage is specifically speaking about Jerusalem and Judah, and in a more general sense, Israel. But I am not Jewish so I don't have to worry about that, right? Wrong! It is the same LORD or JHVH that spoke through Isaiah to the Jewish nation that speaks to the church in general and to the individual specifically. Jesus said in Matthew 10:28, "And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell." The New Testament had not been written yet but Jesus' words to the Jews in His earthly days ought to be taken seriously by us too. We are to fear the LORD because He does have power over both death and the final destiny of mankind. Any who takes God for granted is not wise.
As I reread Isaiah 33 I looked at the full context of the scripture. The thought really starts in Isaiah 33:13 and goes through verse 16. Verse 13 says, "Hear, ye that are far off, what I have done; and, ye that are near, acknowledge my might." God in His mercy is trying to get our attention. Who is going to not be afraid of the, "...devouring fire..." and of the "...everlasting burnings?" The Bible says in verse 15, "He that walketh righteously, and speaketh uprightly; he that despiseth the gain of oppressions, that shaketh his hands from holding of bribes, that stoppeth his ears from hearing of blood, and shutteth his eyes from seeing evil..." It is him that will be able to, "...dwell with the devouring fire? who ...shall dwell with everlasting burnings."
I certainly don't claim to have the last word on these scriptures but when I read about the devouring fire and everlasting burnings, my mind immediately went to the three Hebrew Children of the Book of Daniel and the burning fiery furnace they were thrown into. The Bible says that king Nebuchadnezzar saw four men walking in the flame and the fourth was like the Son of God. (See Daniel 3:24-25) Is it possible that the fire that destroyed Nadab and Abihu in Leviticus 10:1-2, is the same fire that kept the Hebrew Children from harm as they walked with that fourth man? I for one don't want to be a hypocrite and be surprised at judgment. If I will pay heed to the Word of God as Isaiah 33:17 says, I shall, "...see the king in his beauty..." I don't want to wake up and find that I am without excuse. I feel in the Lord Jesus to warn you, whoever you are, that judgment day is coming. Don't wait too long to get right with God.