I believe my understanding is correct that during the Millennium, believers can lose their salvation and will be required to conduct animal sacrifices. But once the Millennium is over and Satan is forever shut away to no longer influence believers, will it still be possible for those living on Earth to lose their salvation? Will they be required to continue animal sacrifices in eternity?
It’s clear that Israel will be required to conduct regular sacrifices for sin during the Millennium (Ezekiel 40-48). What isn’t clear at this point is how the rest of the nations of the world will meet this requirement. Zechariah 14:16-17 says all nations will have to visit Israel once each year for the Feast of Tabernacles, but that’s all we know about it.
Except for telling us there is one, the Bible doesn’t say anything about eternity. However, from 1 Cor. 15:20-28 you can make the case that once the Millennium ends and the Great White Throne judgment has taken place, there will be no sinners left. In fact mankind will no longer have the ability to sin. Therefore, there won’t be any ongoing need for sacrifices to cover their sins.
Remember this all began with Adam and Eve sinning and being banished from the garden of Eden because God could not allow them to live forever as sinners (Genesis 3:22-23). Over the last 6,000 years and for 1,000 years yet to come, God’s single objective has been and will be to reverse the consequences of their sin without destroying all of us in the process, so He can dwell with us again the way He once did with Adam and Eve. Eternity cannot begin until He has accomplished that.