As there will be sin on earth during the Millennium, what is the “process” for being saved: for the forgiveness of sin? With Jesus reigning from Jerusalem, and the likelihood of members of the Church visiting, or even teaching, or policing, on earth during the Millennium, how is it possible that some mortals will not be saved during that time? How can they fail to believe and be saved?
The gift of Salvation will be given, as it always has been, through belief in the atoning death of the Messiah. After the Rapture and in the Millennium, this will be evidenced by obedience to the commandments. For those outside of Israel, life will be similar to to the way it was in Old Testament times. They practiced the Jewish religion at home and made annual pilgrimages to the Holy Land. Zechariah 14:16-17 says that at least one pilgrimage each year, to the Feast of Tabernacles, will be mandatory during the Millennium.
There really isn’t anything in the Bible that describes the Church’s official involvement with happenings on Earth. Yes we will rule and reign, but as the Bride of Christ I think of us in a role more like that of a Queen than of a cadre of officials or administrators. Time will tell.
People will fail to be saved just like they always have, by refusing to accept the gift God has offered. The Scriptures were full of references to the Messiah before He came, but many refused to believe it. Then He came and dwelt in their midst performing amazing miracles and still many refused to believe. There are many more prophecies of his 2nd Coming, and even after He fulfills those and dwells among man again, many will still reject Him.
The evidence we have of this is that following the 2nd coming, the judgments Jesus will conduct will remove every living nonbeliever from earth (Matt. 24:45-25:46). Therefore at the outset of the Millennium there will be only believers left to repopulate the planet. Fast forward to the end of the Millennium and we discover that Satan will be able to recruit a massive army with which to attack Jerusalem (Rev. 20:7-9). These recruits will come from those who will have been born during the Millennium, but will have rejected the pardon Jesus purchased for them on the cross.