For more than sixteen centuries the Lord dealt exclusively with the Jewish nation. They were favored above every other nation in the earth. The Lord told them on one occasion that He recognized them only of all the families of the earth (Amos 3:2). The Israelites were informed that if faithful to the Lord, and obedient to His commands, they would be a chosen generation, a royal priesthood, and that His favor would never depart from them. But they became disobedient to the Lord’s instructions and consequently, they were cast aside and no longer recognized as His people. 

The Divine favor was then turned to the Gentiles and these were granted the opportunity to become members of the Royal Priesthood. This Priesthood would have been completely made up from the twelve tribes of Israel–twelve thousand from each tribe–had there been a sufficient number of faithful obedient ones to make up the predestinated number of 144,000. As only a remnant of that nation of the Jews accepted Christ as their Savior, the invitation went to the Gentiles. All throughout this Gospel Age the selection has been going on amongst the nations of the earth. When “the fullness (the full number) of the Gentiles be come in,” then favor will once more return to the Jews and all Israel shall be saved (Romans 11:25-27). The elect 144,000 will be kings and priests unto God and shall reign with Christ during the thousand years mentioned in Revelation 20:1-6.

Additional Resources: 
Christian Questions Podcast Episode 1248: “What Will We Be Doing When We Get to Heaven? (Part I)”
Examining the privilege of heaven, why it is open and how to get there
Preview Video  CQ Rewind Show Notes

Christian Questions Blog: Who is part of the 144,000 in Revelation?