Thank you, my dear friend, for such an excellent question! No doubt you are thinking about such NT verses as Hebrews 10:12-18 where we read: “But this Man, after He had offered one sacrifice for sins forever, sat down at the right hand of God, from that time waiting till His enemies are made His footstool. For by one offering He has perfected forever those who are being sanctified…. Now where there is remission of these, there is no longer an offering for sin.” (NKJV). And yet, in Ezekiel 44-46 we hear that during the 1000-year reign of Christ, the temple sacrifices will be resumed, and some of these are designated to be for “atonement”. I assume then that your question is that since Christ Jesus has made atonement at Calvary’s cross, and this once and for all, why in the millennium would there be any need for further atonement? I will defer to the MacDonald Bible Commentary for a few of his comments on Ezekiel 45-46 because I believe his explanation is both accurate and very understandable: “As used in the OT, the word “atonement” (lit., covering) never means the putting away of sins. Hebrews 10:4 reminds us that “… it is not possible that the blood of bulls and goats could take away sins.” Rather the sacrifices were an annual reminder of sins (Heb. 10:3). What then did atonement mean? It meant that the sacrifices produced an outward, ceremonial cleanness. They conferred a ritual purification on people, enabling them to draw near as worshipers in fellowship with God. The sacrifices even made atonement for inanimate things, such as the altar (Ex. 29:37), where there could be no thought of remission of sins. All it means is that the altar was cleansed ceremonially and thus made fit for God’s service…. When we read of the forgiveness of unintentional sin in connection with atonement (Lev. 4:20), it can only mean the removal of ceremonial defilement so that the person could draw near in worship. In our day the word “atonement” has acquired a much wider and deeper meaning. It is used, for instance, to describe the entire sacrificial work of Christ by which our sins are put away and we are reconciled to God. But it never has this meaning in the Bible. (In Rom. 5:11 KJV, the word “atonement” should be “reconciliation,” as in NKJV and other versions.) The sacrifices in Israel’s history looked forward to the perfect and complete sacrifice of Christ. The sacrifices in the Millennium will commemorate His work on Calvary. They will be memorials for Israel just as the Lord’s Supper is for us.

The passages in Hebrews do not rule out any sacrificial ceremony in the future. But they insist that no future sacrifices can ever deal effectively with sins, any more than they did in the past.”

In summary, I believe that we must remember that the OT sacrifices pointed forward to Christ.  Christ, at Calvary, shed His precious blood in order to make atonement for our sins, and He being the very Son of God, was the only One who could accomplish this for those who believe on Him (Hebrews 10:14). Furthermore, by the time of the Great Tribulation, which precedes the millennium, the church, the body of all true believers on the Lord Jesus Christ, will have been taken up to be with the Lord (1 Thessalonians 4:16-18; Revelation 3:10). At the end of this Great Tribulation, Christ Jesus will return the second time to the earth to judge the nations and to defeat all His enemies, those that are in rebellion against God (Malachi 3:1-3; Zechariah 14). Then, Christ will set up His millennial reign in Jerusalem on the throne of David. Ezekiel 45 and 46, tells of this millennial time very specifically. In that day, the temple worship will be re-established, and the faithful remnant of Israel will be returned to their land from all over the earth, and the nation of Israel will be restored with much blessing. With the temple rebuilt, the temple worship will then resume. And yes, there will be animal sacrifices during this time, but rather than to be an atonement for sins, these sacrifices will serve to remind the faithful of Israel, secure and blessed in the land God promised them, of the sacrifice of their Lord and King, Jesus Christ, which has brought them into this time of earthly blessing.  Now, I think you’ll find this interesting: The church (Christians) today does not observe the OT feasts and temple rituals, but in the millennium, the faithful of Israel will resume at least some of these. Zechariah 14 speaks of the coming again to the earth of Christ, the Messiah of Israel,  and in verse 16 of that chapter, we see a prophetic word regarding one of the feasts of Jehovah called for in Leviticus 23, the Feast of Tabernacles “And it shall come to pass that everyone who is left of all the nations which came against Jerusalem shall go up from year to year to worship the King, the LORD of hosts, and to keep the Feast of Tabernacles.” In this prophesy, this feast is the only one of the required feasts for Israel under the OT Law mentioned, and it will be celebrated during the millennium. In the book The Feasts of Jehovah, William Kelly states: “…it helps us to understand how the children of Israel will be brought into their blessedness. There were to be “seven days” of suffering in grace, as we have them now under the type of the Unleavened bread; and seven days of glory in the age to come. This will be the Feast of Tabernacles in its ordinary character for Israel on earth.” (SF)  (648.5)