Ezekiel 44:18

They shall have linen bonnets upon their heads, and shall have linen breeches upon their loins; they shall not gird themselves with any thing that causeth sweat.

The verse directs the priests in the vision to wear linen coverings on their heads and linen garments on their loins, and to avoid girding themselves with anything that causes sweat. It belongs to the closing vision of Ezekiel 40-48 concerning the sanctuary and its offerings and sacrifices, its glory, the city, the division of the land, and the service of the priests. What was shown was conditional on Israel being ashamed of its iniquities and putting them away, repenting and keeping God's ordinances, and because Israel did not meet the conditions it was forfeited and never built as described. It will never be built literally in the future, for renewing its thousands of animal sacrifices would make Christ's offering once for all of no effect and return from sunlight to shadow. In its spiritual aspect it stands as a faint type to be fulfilled in general at Christ's second coming and in the new heavens and new earth for true Israel, with God dwelling in glory, healing waters, lengthened life, absolute peace, and unsurpassable joy.

Place in Ezekiel's Vision

They shall wear linen caps on their heads and linen trousers on their loins, linen garments for priestly service, and shall not belt themselves with anything that makes them sweat.
The rule stands in the temple-city vision of Ezekiel 40-48, and in its closing chapters generally, concerning the sanctuary and its offerings and sacrifices, its glory, the city, the division of the land, and the service of its priests.
Those surrounding instructions include literal circumcision, distinctions of clean and unclean foods, and Levitical offerings and priesthood.
No such city or sanctuary stands today, so the law of priestly dress has not been fulfilled.

The vision beginning in Ezekiel 40 sets forth God's purpose for Jerusalem and is the counterpart of the vision of the man with the measuring line in Zechariah 2, the two covering the same period and representing the same purposes and to be studied together.
It offered a literal, mortal sanctuary of the past Mosaic dispensation under the first covenant, not the immortal state nor the gospel dispensation.
Its priests in linen dress are all Levites, not a people gathered out of all nations to be priests in a future age.
Their service belongs to a typical shadow system, with its thousands of sheep and oxen sacrifices for the time then present shadowing forth the only effectual offering of Jesus Christ, which cannot be revived after Christ's heavenly ministration closes.
To make that vision a literal city to be built on this sin-cursed earth during the thousand-year reign, with its offerings and sacrifices carried on again, is to return to the ordinances of the first covenant and to set up ancient Jerusalem in place of the heavenly Jerusalem coming down from God out of heaven.

Conditional Promise Unfulfilled

The promise of the sanctuary and priestly service in Ezekiel 44, including the linen bonnets and breeches and the prohibition of anything causing sweat, was conditional on Israel being ashamed of iniquities and putting them away, repenting, reforming, yielding fully to God and keeping His ordinances as stated in Ezekiel 43:9-11 and on the principle of Jeremiah 18:7-10. Because Israel did not meet those conditions it was forfeited and the sanctuary described in Ezekiel 40-48 was never built as described. The description belonged to an order intended for the past dispensation when its prescribed ordinances were in force, and Israel's unfaithfulness prevented that promise from being fulfilled as described. What would have been built if Israel had been faithful remains instead, in its spiritual aspect, a prophecy of greater things the Lord would do for His people through Christ.

The vision was given during the Babylonian captivity, twenty-five years into it, holding out what God would have done at the restoration from Babylon at the end of the seventy years. It showed the city and temple as they would doubtless have been built on earth, a house of prayer for all nations after exile and dispersion, if all Israel had responded to the three calls to come out of Babylon, returned wholeheartedly, yielded to God, been ashamed, and truly and fully repented. That promised glory after the return from Babylon — restoration to the land and rebuilding of the temple — was conditional and was never inherited because the stated condition was disregarded. Because Israel according to the flesh did not yield, its literal details will never be fulfilled, though the promises concerning true Israel will still be fulfilled not in the described details but according to later revelations.

The glorious temple Ezekiel described was not built, and only a temple with a shadow of its greatness was erected by Zerubbabel, with others after it only a shadow falling far short of the pattern. That temple built at the restoration from Babylon fell far short of Ezekiel's pattern and never saw the glory of Solomon's temple except in the person of Jesus Christ. This was because Israel remained apostate and half-hearted, would not yield all to God for Him to come in and dwell among them, and Babylon was dearer to Israel than the Jerusalem they professed to love. God would not build a temple for His name for a half-hearted people to dishonor, and the city shown to Ezekiel was never built, not because God changed His mind but because Israel failed to do their part. Because the people refused to heed the prophecy, only a mere shadow of the temple was built instead of the structure shown through Ezekiel.

It will never be fulfilled literally or built in the future, for no such city or sanctuary stands today and its priestly dress law has not been fulfilled. Renewing the thousands of sheep and oxen sacrifices of that sanctuary system would make the offering of Christ once for all of no effect, building again the things destroyed and returning from sunlight to shadow. Those offerings were for the time then present, shadowing forth the only effectual offering, that of Jesus Christ, and by His death on the cross He fulfilled all sacrifices. To revive that typical shadow system after Christ's heavenly ministration closes cannot be done, and doing that work again would require a second Messiah at the end of the thousand years to expiate transgressions. The priests it describes in linen dress are all Levites, not a people gathered out of all nations to be priests in a future age.

The vision of the city and sanctuary in Ezekiel 40-48 is not the New Jerusalem of Revelation; similarity does not prove identity and points of dissimilarity forbid identity. It is not to be made a literal city on this sin-cursed earth during the thousand-year reign of Christ, with its offerings and sacrifices carried on again, which would return to the ordinances of the first covenant and set up ancient Jerusalem in place of the heavenly Jerusalem coming down from God out of heaven. Such a state is less glorious than the present dispensation, since the offerings of the first covenant pointed to Christ the great sin-offering, and by one offering He perfected forever them that are sanctified.

In its spiritual aspect the vision stands as a faint type to be fulfilled in general rather than in every detail at Christ's second coming and in the new heavens and new earth for true Israel and the faithful, with former local details eliminated. Wherein it failed for old earthly Jerusalem it will be grandly realized in the New City that awaits the true Israel of God, and the future glory and fulness of the immortal inheritance will come to the faithful in fullest measure, more glorious than what was offered then. Then God dwelling in glory, healing waters and life and healing power for barren land and marshes and impure waters, lengthened life, subjection of enemies far and near including the kings of the North, raising of the righteous dead, absolute peace and harmony, and unsurpassable joy will be restored by His mighty power. The vision beginning in Ezekiel 40 is the counterpart of the vision of the man with the measuring line in Zechariah 2, covering the same period and representing the same purposes, to be studied together.

Because Israel failed to do its part, in order to have a holy city God had to prepare it in heaven, that city is prepared and will descend upon the earth. The ancient calls to come out of Babylon were an object-lesson of the last call now sounding through the earth, and utter destruction awaits those who linger in Babylon today.