The Centerpiece of God’s Eternal Plan:
“Making known to us the mystery (secret) of His will (of His plan, of His purpose). [And it is this:] In accordance with His good pleasure (His merciful intention) which He had previously purposed and set forth in Him,  [He planned] for the maturity of times and the climax of the ages to unify all things and head them up and consummate them in Christ, [both] things in heaven and things on earth.”
(Ephesians 1:9-10 AMP)
1. The centerpiece of God’s eternal plan is Jesus’ return to earth.
2. Establish His Kingdom rule.
3. Joining the heavenly and earthly realms together. Both realms must come together to express God’s personality and purposes.
The spirit realm and the material realm coming together:
In the image above, we see the New Jerusalem descending to just above the Millennial City; “THE LORD IS THERE“ (Ezk. 48:35; Rev. 21:12, 13, 16). The two Jerusalem s will be connected by the “Corridor of Glory”.
Stage one: The descent of the New Jerusalem is partial yet substantial in the Second Coming of Christ.
Stage two: The descent of the New Jerusalem is total and ultimate after the Millennium.
Where will the saints live?
The residences of the “saints” will be in the New Jerusalem and they will rule with governmental responsibility on earth. Kind of like senators living in their home state; then a government office in Wash., DC.
“But we are citizens of the state (commonwealth, homeland) which is in heaven, and from it also we earnestly and patiently await [the coming of] the Lord Jesus Christ (the Messiah) [as] Savior.” 
“Who will transform and fashion a new body of our humiliation to conform to and be like the body of His glory and majesty, by exerting that power which enables Him even to subject everything to Himself.” 
~ Philippians 3:20-21 AMP
What kind of body will the “saints” have?
According to the passage in Philippians, the bodies will be “transformed and fashioned anew“. They will have a solid material body with supernatural abilities.
“On saying this, she turned around and saw Jesus standing [there], but she did not know (recognize) that it was Jesus. Jesus said to her, Women, why are you crying [so]? For Whom are you looking? Supposing that it was the gardener” (John 20:14-15 AMP).
“Morning was already breaking when Jesus came to the beach and stood there. However, the disciples did not know that it was Jesus. So Jesus said to them, ‘Boys (children), you do not have ant meat (fish), do you?” (John 21:4-5 AMP).
Jesus spoke with His disciples for forty days in His resurrected body (Acts 1:3). Sometimes spirit beings (demons and angels) do appear in the natural realm to interact with people in natural bodies.
The resurrected saints will occupy the highest positions of government in the Spirit; in the Millennial earth. Parallel yet a superior relationship of angels to demonic principalities at this present time (Eph. 6:12; Daniel 10:12-13).
Why will priests be needed in the Millennial earth?
“Natural people” still have a sin nature which causes them to be selfish and sinful. They will live in Paradise and surrounded by bounty and yet still find reasons to complain. Sound familiar to those around us today on earth?
The presence of the “natural” have survived the Tribulation, the Battle of Armageddon and judged righteous. They will experience the same life cycle of all mortals. They will:
“There shall no more be in it an infant who lives but a few days, or an old man who dies prematurely; for the child shall die a hundred years old, and the sinner who dies ONLY a hundred years old shall be [thought only a child, cut off because he is] accursed” 
“They shall build houses and inhabit them, and they shall plant vineyards and eat fruit of them” 
“They shall not build and inhabit; they shall not plant and another eat [the fruit]. For as the days of a tree, so shall be the days of My people, and My chosen and elect shall long make use of and enjoy the work of their hands” 
“They shall not labor in vain or bring forth [children] for sudden terror or calamity; for they shall be the descendants of the blessed of the Lord, and their offspring with them” 
“And it shall be that before they call I will answer, and while they are yet speaking I will hear” 
“The wolf and the lamb shall feed together, and the lion shall eat straw like the ox, and dust shall be the serpent’s food. They shall not hurt or destroy in all My holy Mount [Zion], says the Lord” 
~ Isaiah 65:20-25 AMP.
“Natural people” in the Millennium Kingdom explains the need of priests (Ezekiel 42:13, 14; 44:15-13; Revelation 5:10).
A priest, by definition, is someone who intercedes or mediates between God and another person. If everyone in the Millennial Kingdom had a new, everlasting, regenerated and intimate relationship with God, there would be no need for priests. Since there will be many “natural people“, priests will be needed.
Death will be rare on the earth, for one who dies at 100 years of age will be considered a youngster.
You might be asking yourself, “How about that verse in the Book of Psalms about living to the ripe old age of 70 or 80?” You are referring to Psalm 90:10 most likely, A Prayer of Moses the man of God:
“The days of our years are threescore years and ten (seventy years)–or even, if by reason of strength, fourscore years (eighty years)”
The footnote in the Amplified Bible says that, “This psalm is credited to Moses, who is interceding with God to remove the curse which made it necessary for every Israelite over twenty years of age (when they rebelled against God at Kadesh-Barnea) to die before reaching the promised land (Num. 14:26-35). Moses says most of them are dying at seventy years of age. This number has often been mistaken as a set span of life for all mankind. It was not intended to refer to anyone except those Israelites under the curse during that particular forty years. Seventy years never has been the average span of life for “natural man” (humanity). When Jacob, the father of the twelve tribes, had reached 130 years (Gen. 47:9), he complained that he had not attained to the years of his immediate ancestors. In fact, Moses himself lived 120 years old. Aaron 123, Miriam several years older, and Joshua 110 years of age. Note as well in the Millennium a person dying at 100 will still be thought a child (Isa. 65:20).”
Whatever your position or thinking might be regarding the Tribulation or the Millennial Kingdom coming is not the point of this blog. The only point is to present you with the evidence found in my detective investigation. I look forward to your comments.