Interesting exchange between Myelaine and Tec. I knew it was only a matter of time before this got brought up. These kinds of questions are going to be common among JW's and ex-Jw's. So, we might as well grapple with them now.
I'll try to succiently state the issue: If we are saved by the blood of the covenant and not by works, then how is it that the sheep are allowed into the Milenium based on how they treated Christ's brothers? Though not brought up by Meelaine and Tammy, one could also ask how Jesus could make the statement: "the one who has endured to the end is the one who will be saved".
1.) First let me state that all of these statements are true. To understand, we're going to have to adopt the 30,000 ft. view instead of the 25ft. view. To do so, there is no better way than to look at Daniels' 70 week prophecy as a template.
2.) We also need to understand the definition of what it means to be saved, and what we are saved from. And, can we be saved from more than one terrible fate? We also need to understand what a sheep is and if a sheep can be in a positionally different condition than a saved person.
But even before this, we need to understand that the church age is a great parentheses in the plan of God that he started with mankind beginning with Abraham. In Old Testament times, the church age was a mystery and was not seen by the OT prophets, though there are a few vague allusions to it. The prophets (and Jesus was one of them) spoke primarily to the Jews, not to the gentiles. Like the chart below illustrates they spoke about current events and could see into the future Milenium and also knew about Judgment Day. The Church age (the age of grace) was not seen and was not generally spoken about (there are exceptions though). In the chart below, the prophets saw and wrote about the peaks, but the valley of the church age was generally not.
We know that 69 weeks of years in Daniels' prophecy elapsed, and exactly on time the Messiah appeared. But then the prophetic clock stopped. This ushered in the age of Grace, the church age - which was unseen by the Old Testament prophets. But, what about the 70th week? God still owes the Jews one more week, the final week. This final week will begin with a Peace Agreement broked by the Anit-Christ with modern day Israel. At that time the Age of Grace (the Church Age) will be over.
God dealt with the nation of Israel based on what they did; similarly during this final week what a person does will also imact their destiny during the final 'week' of Daniel's prophecy. Apparently, how a person treats the Jews (Christ's brothers) during this time will determine whether or not they are allowed to enter into the Milenium. If they weren't already, probably at this time the ones allowed to enter the Millenium will be "saved" by faith in the blood of the covenant from not only this judgment of who gets to enter the Millenium, but also from the Great White Throne judgment of the unsaved at the end of the Millenium.
Now, we know that there will be sin and death in the Millenium, and at the same time people will live to great ages:
People who live during the Millennium will not die young. A person one hundred years old will be considered a child, since, presumably, a person could live throughout the 1000 years in the restored environment (Isaiah 65:20-23). Sickness will not be a problem because "The sun of righteousness shall arise with healing in its wings" (Malachi 4:2). Later yet, after the New Earth is established, the Tree of Life will be accessible to mankind. Its leaves are "for the healing of the nations" (Revelation 22:2).
So, probably some of the children of the original subjects of the Millenium would later choose to sin and would somhow die during that time. While I do not think that the sheep who are allowed to enter have glorified bodies like the church age saints have who were by that time previously raptured, one thing is for sure - they are still declared righteous by their faith in the finished work of Christ on the Cross and the blood covenant.
So what I'm saying is that while treating Christ's Brothers (the Jews) kindly during the Great Tribulation might get you entrance into the Millenium (if you are lucky enough to survive until the end), it won't be enough to spare you from the Great White Throne Judgment at the end of the Millenium.
Whole books could be written on this subject, and there are acceptable variations of the above template I've laid out. But, I believe the above represents a sampling of mainstraem conservative and historical Christian thought on this matter.