If hell is eternal, why did the Lord contrast it with the kingdom—which is limited to 1000 years in time?
This question serves as an excellent example of the vital importance of "cutting in a straight line the word of truth" (2 Tim. 2: 15)—in this case of not reading the present into the past.
The testimony of the Lord Jesus was that "I have not been sent save to the lost sheep of Israel’s house" (Matt. 15: 24). Accordingly, on each of the 11 times hell is spoken about in the Gospels (see Matt. 5: 22, 29, 30; 10: 28; 18: 9; 23: 15, 33; Mark 9: 43, 45, 47; Luke 12: 5) the audience is Jewish and the subject is the kingdom. Hell is also referred to in James 3: 6, but James was probably the first epistle to be written and was addressed "to the twelve tribes" (v1) while the establishment of the kingdom was still a possibility. References elsewhere in the NT to hell (for example in the AV of Acts 2: 27) are better translated as hades.
Now in the Synoptic Gospels the Lord’s ministry was based on what had been previously revealed in the OT. Apart from Is. 65: 17; 66: 22, the eternal state is not mentioned in the OT and all future life and blessing for the Jew was identified with the promised kingdom on earth in time and nothing else. The Lord Himself confirmed this, as can be seen by comparing "And if thy foot serve as a snare to thee, cut it off: it is better for thee to enter into life lame, than having thy two feet to be cast into hell, into the fire unquenchable" with "And if thine eye serve as a snare to thee, cast it out: it is better for thee to enter into the kingdom of God with one eye, rather than having two eyes to be cast into the hell of fire" (Mark 9: 45, 47, my emphasis). This kingdom was proclaimed by John the Baptist (see Matt. 3: 2), the Lord Himself (see Matt. 4: 17) and the twelve apostles (see Matt. 10: 7). No one ever questioned what that kingdom was, for the Jews knew that it was the kingdom prophesied by Daniel. They knew that "the God of the heavens" would "set up a kingdom which shall never be destroyed; and the sovereignty thereof shall not be left to another people" (Dan. 2: 44). They were well aware that "His kingdom is an everlasting kingdom" (Dan. 7: 27) and that the saints "will possess the kingdom for ever, even to the ages of ages" (v18). However, there was no thought whatsoever then of the period of that kingdom being limited to 1000 years (the kingdom continues eternally, but not on this present earth). The kingdom being 1000 years long awaited a future revelation given to John the apostle (see Rev. 20: 2–7). Hence, we must not read this fact, now revealed, into the ministry of the Lord Jesus. If we do we will read the present into the past and get ourselves into difficulties. But all this raises other questions: Why is the word hell absent from the rest of the NT? Why is it called "the lake of fire" in the Revelation?
The only alternative to blessing in the kingdom that the Lord presented to the Jew was encompassed in the word hell (in Greek Gehenna). Gehenna is the Greek translation of two Hebrew words for the valley of Hinnom. This place was where king Ahaz and king Manasseh, along with idolatrous Israel, sacrificed children by fire to Molech (see 2 Chron. 28: 3; 33: 6). In response, the godly king Josiah defiled the valley (see 2 Kings 23:10) and after the return of the Jews from captivity it was held in such abhorrence that it was made the place where all the dead carcases and offal of Jerusalem were thrown. To avoid pollution and disease, it became necessary to keep fires continually burning there. It thus provided a fitting figure of the "eternal fire" of hell (Matt. 18: 8) "where their worm dies not, and the fire is not quenched" (Mark 9: 48). The thought of "where their worm dies not" occurs only here and in the last verse of Isaiah (Is. 66: 24)—from which it is likely to have been taken. The Lord’s hearers would have been familiar with the reference.
But why is the word "hell" absent from the rest of the NT? A comparison of Matt. 5: 30 and Matt. 18: 8 shows that "hell" in Matt. 5: 30 is the same as the "eternal fire" of Matt. 18: 8 (the words "cut it off" occur in both passages). From Matt. 25: 41, we also know that this "eternal fire" was prepared for the devil and his angels, and it must therefore be the same as the "the lake of fire" in Rev. 20: 10 into which Satan is cast. Hence the words "hell" and "the lake of fire" describe one and the same place. Now while the imagery of the word Gehenna (hell) had great significance to the Jews addressed by the Lord, it could have no significance whatsoever to Gentiles of that day. Only the Jews knew the significance of the valley of Hinnom. Hence when John penned the Revelation, he used terminology, namely "the lake of fire", that all, Jew and Gentile, could understand.