The Bible testifies that saints saved in New Testament times are made kings and priests. 

[Revelation 5:10] And hast made us unto our God kings and priests: and we shall reign on the earth.

Then who are the saved people in the New Testament age? According to the evidence of the Bible, the number of people who have been saved is one hundred and forty-four thousand. The Bible testifies about this as follows. 

[Revelation 14:3~4] And they sang a new song before the throne and before the four living creatures and the elders. No one could learn the song except the 144,000 who had been redeemed from the earth. These are those who did not defile themselves with women, for they kept themselves pure. They follow the Lamb wherever he goes. They were purchased from among men and offered as firstfruits to God and the Lamb. 

In the Bible, it's the 144000 who are saved, and these are priests who intercede for the people, not the general population.

But if we look at their characteristics, they are described as firstfruits. So why are the 144000 described as firstfruits? Firstfruits is a prophecy of the firstborn in the Old Testament law, so if we study about the firstborn in the law, we will know more about the 144000. In the Old Testament law, there is a law of the firstborn, which means that the firstborn of man and beast belong to God. 

[Exodus 13:11~16] After the LORD brings you into the land of the Canaanites and gives it to you, as he promised on oath to you and your forefathers, you are to give over to the LORD the first offspring of every womb. All the firstborn males of your livestock belong to the LORD. Redeem with a lamb every firstborn donkey, but if you do not redeem it, break its neck. Redeem every firstborn among your sons. "In days to come, when your son asks you, 'What does this mean?' say to him, 'With a mighty hand the LORD brought us out of Egypt, out of the land of slavery. When Pharaoh stubbornly refused to let us go, the LORD killed every firstborn in Egypt, both man and animal. This is why I sacrifice to the LORD the first male offspring of every womb and redeem each of my firstborn sons.' And it will be like a sign on your hand and a symbol on your forehead that the LORD brought us out of Egypt with his mighty hand." 

When God freed Israel from Egypt, He killed all the firstborn in the land of Egypt, both human and animal, but protected the firstborn of the people and animals that belonged to Israel. To commemorate this, He established a law that attributed the firstborn of Israel to God.However, God did not take all the firstborn of the 12 tribes of Israel, but chose the tribe of Levi to be the firstborn of Israel. 

[Numbers 8:16~19] They are the Israelites who are to be given wholly to me. I have taken them as my own in place of the firstborn, the first male offspring from every Israelite woman. Every firstborn male in Israel, whether man or animal, is mine. When I struck down all the firstborn in Egypt, I set them apart for myself. And I have taken the Levites in place of all the firstborn sons in Israel. Of all the Israelites, I have given the Levites as gifts to Aaron and his sons to do the work at the Tent of Meeting on behalf of the Israelites and to make atonement for them so that no plague will strike the Israelites when they go near the sanctuary." 

The reason God chose the Levites to be the firstborn was to help Aaron and his descendants perform the sacrifices, so the office of firstborn means the office of priest. 

Therefore, the 144,000 who are saved as firstfruits are those who have received the office of priests. They were priests who served God, which is why He refers to them as kingly priests. In the Old Testament, the tribe of Levi was appointed as the firstborn to fulfill the priestly office, but in the New Testament, He chooses 12,000 people from each tribe to make 144,000 priests.

 In the Bible, many people are saved besides the 144,000, but these are prophecies about the general people. There were about 10 million people in Israel, made up of 12 tribes, of which the tribe of Levi performed the duties of the priesthood. So the people in white robes who appeared as a great crowd are the general people, and the 144,000 saints are the priesthood. Just as the Old Testament priests assisted Aaron and Aaron's sons in the service of the temple, so the New Testament priests assist Joshua and the two witnesses in the service of the priesthood.