The root of the problem here is the Biblical meaning of the words 'eternal fire', 'for ever', and the various forms of 'unquenchable', i.e. 'not be quenched'. Most people, understandably so, misunderstand the Biblical concept of these terms. In the Bible these terms sometimes do and sometimes dont equate to our modern meaning of "forever". In today's usage these terms mean 'for the ceaseless ages of eternity' for the most part though not exclusively. For example:
A married couple: they tell each other "I'll love you forever" but we all know that people die. What they are truly saying is 'they love each other until death', right? This is a parallel to see how even in modern times "forever" doesnt necessarily mean the ceaseless age of eternity.
So, let's go over what the Bible writers concept of forever was:
Genesis 43:9 - I will be surety for him; of my hand shalt thou require him: if I bring him not unto thee, and set him before thee, then let me bear the blame for ever:
Genesis 44:32 - For thy servant became surety for the lad unto my father, saying, If I bring him not unto thee, then I shall bear the blame to my father for ever.
This was pre-incarnate Jesus talking to the Father in regards to mans sin. Now, will Jesus bear the blame forever? No. (see Hebrews 9:28) The day will come when there is an end to sin. So the meaning here is clearly meant as until it is done.
Exodus 12:14 - And this day shall be unto you for a memorial; and ye shall keep it a feast to the LORD throughout your generations; ye shall keep it a feast by an ordinance for ever.
Exodus 12:17 - And ye shall observe the feast of unleavened bread; for in this selfsame day have I brought your armies out of the land of Egypt: therefore shall ye observe this day in your generations by an ordinance for ever.
Exodus 12:24 - And ye shall observe this thing for an ordinance to thee and to thy sons for ever.
But we know that the feasts were done away with when the Old Covenant was fulfilled by Christ. Exodus 12:14 is talking about the institution of the Passover; later Jesus became our Passover (1 Corinthians 5:7). The key in this verse is the part that says by an ordinance. We know that the ordinances were nailed to the cross (Colossians 2:14); and there arent any ordinances in the 10 Commandments. So again we find that forever does not mean the ceaseless ages of eternity as is commonly misunderstood.
Let's get some more examples:
Exodus 19:9 - And the LORD said unto Moses, Lo, I come unto thee in a thick cloud, that the people may hear when I speak with thee, and believe thee for ever. And Moses told the words of the people unto the LORD.
The question here is, how can the people believe thee for ever if they were mere mortals? They could not, of course, unless they were saved and would get ressurected at Christ's second coming. They died and the dead know not anything (Ecclesiastes 9:5). The meaning here is that they would believe for as long as they lived, not the ceaseless ages of eternity. Also, the thick cloud that could be seen by day (it was a fire from the sky at night) was no longer seen after Moses death. So, again, for ever does not equate to our modern understanding of forever.
Exodus 21:6 - Then his master shall bring him unto the judges; he shall also bring him to the door, or unto the door post; and his master shall bore his ear through with an aul; and he shall serve him for ever.
Would it be possible for a servant or master to live forever? No. Is a man still a servant to another after death? No. This clearly means that the servant would be a servant for the rest of his life and not the ceaseless ages of eternity.
Exodus 27:21 - In the tabernacle of the congregation without the vail, which is before the testimony, Aaron and his sons shall order it from evening to morning before the LORD: it shall be a statute for ever unto their generations on the behalf of the children of Israel.
Exodus 28:43 - And they shall be upon Aaron, and upon his sons, when they come in unto the tabernacle of the congregation, or when they come near unto the altar to minister in the holy place; that they bear not iniquity, and die: it shall be a statute for ever unto him and his seed after him.
Did the earthly tabernacle and its ceremonies last forever? No. (see Col. 2:14) Again, we find the meaning of for ever to mean until it is done.
In the case of man, this means "as long as he lives" or "until death." (See 1 Samuel 1:22, 28; Exodus 21:6; Psalm 48:14.) So the wicked will burn in the fire as long as they live, or until death. This fiery punishment for sin will vary according to the degree of sins for each individual, but after the punishment, the fire will go out; it will not last for the ceaseless ages of eternity.
Jeremiah 17:27 - But if ye will not hearken unto me to hallow the sabbath day, and not to bear a burden, even entering in at the gates of Jerusalem on the sabbath day; then will I kindle a fire in the gates thereof, and it shall devour the palaces of Jerusalem, and it shall not be quenched.
We all know that Jerusalem is not still burning today; as such the fire that 'shall not be quenched' means that it cannot be put out UNTIL it has done God's purpose. No man can quench it; only God can. And God obviously did quench that fire else it would still be burning to this day.
A married couple: they tell each other "I'll love you forever" but we all know that people die. What they are truly saying is 'they love each other until death', right? This is a parallel to see how even in modern times "forever" doesnt necessarily mean the ceaseless age of eternity.
So, let's go over what the Bible writers concept of forever was:
Genesis 43:9 - I will be surety for him; of my hand shalt thou require him: if I bring him not unto thee, and set him before thee, then let me bear the blame for ever:
Genesis 44:32 - For thy servant became surety for the lad unto my father, saying, If I bring him not unto thee, then I shall bear the blame to my father for ever.
This was pre-incarnate Jesus talking to the Father in regards to mans sin. Now, will Jesus bear the blame forever? No. (see Hebrews 9:28) The day will come when there is an end to sin. So the meaning here is clearly meant as until it is done.
Exodus 12:14 - And this day shall be unto you for a memorial; and ye shall keep it a feast to the LORD throughout your generations; ye shall keep it a feast by an ordinance for ever.
Exodus 12:17 - And ye shall observe the feast of unleavened bread; for in this selfsame day have I brought your armies out of the land of Egypt: therefore shall ye observe this day in your generations by an ordinance for ever.
Exodus 12:24 - And ye shall observe this thing for an ordinance to thee and to thy sons for ever.
But we know that the feasts were done away with when the Old Covenant was fulfilled by Christ. Exodus 12:14 is talking about the institution of the Passover; later Jesus became our Passover (1 Corinthians 5:7). The key in this verse is the part that says by an ordinance. We know that the ordinances were nailed to the cross (Colossians 2:14); and there arent any ordinances in the 10 Commandments. So again we find that forever does not mean the ceaseless ages of eternity as is commonly misunderstood.
Let's get some more examples:
Exodus 19:9 - And the LORD said unto Moses, Lo, I come unto thee in a thick cloud, that the people may hear when I speak with thee, and believe thee for ever. And Moses told the words of the people unto the LORD.
The question here is, how can the people believe thee for ever if they were mere mortals? They could not, of course, unless they were saved and would get ressurected at Christ's second coming. They died and the dead know not anything (Ecclesiastes 9:5). The meaning here is that they would believe for as long as they lived, not the ceaseless ages of eternity. Also, the thick cloud that could be seen by day (it was a fire from the sky at night) was no longer seen after Moses death. So, again, for ever does not equate to our modern understanding of forever.
Exodus 21:6 - Then his master shall bring him unto the judges; he shall also bring him to the door, or unto the door post; and his master shall bore his ear through with an aul; and he shall serve him for ever.
Would it be possible for a servant or master to live forever? No. Is a man still a servant to another after death? No. This clearly means that the servant would be a servant for the rest of his life and not the ceaseless ages of eternity.
Exodus 27:21 - In the tabernacle of the congregation without the vail, which is before the testimony, Aaron and his sons shall order it from evening to morning before the LORD: it shall be a statute for ever unto their generations on the behalf of the children of Israel.
Exodus 28:43 - And they shall be upon Aaron, and upon his sons, when they come in unto the tabernacle of the congregation, or when they come near unto the altar to minister in the holy place; that they bear not iniquity, and die: it shall be a statute for ever unto him and his seed after him.
Did the earthly tabernacle and its ceremonies last forever? No. (see Col. 2:14) Again, we find the meaning of for ever to mean until it is done.
In the case of man, this means "as long as he lives" or "until death." (See 1 Samuel 1:22, 28; Exodus 21:6; Psalm 48:14.) So the wicked will burn in the fire as long as they live, or until death. This fiery punishment for sin will vary according to the degree of sins for each individual, but after the punishment, the fire will go out; it will not last for the ceaseless ages of eternity.
Jeremiah 17:27 - But if ye will not hearken unto me to hallow the sabbath day, and not to bear a burden, even entering in at the gates of Jerusalem on the sabbath day; then will I kindle a fire in the gates thereof, and it shall devour the palaces of Jerusalem, and it shall not be quenched.
We all know that Jerusalem is not still burning today; as such the fire that 'shall not be quenched' means that it cannot be put out UNTIL it has done God's purpose. No man can quench it; only God can. And God obviously did quench that fire else it would still be burning to this day.