Isa 4:2-3 In that day the Branch of the LORD will be beautiful and glorious, ... Those who are left in Zion, who remain in Jerusalem, will be called holy, all who are recorded among the living in Jerusalem

The Bible teaches us that those who remain in Jerusalem will be called holy. 'Holy' only can be used to God, but if someone is called 'holy' it means the one is one body with God. In other words, it refers that the one is who are to be saved. So people in Isa 4:3 refers to the saints who will be saved. And the Bible says that they stay in Jerusalem. Is this Jerusalem the castle located in Israel? If it is right, we should take airplane, go to Israel, and live in Jerusalem.
Actually there are two kinds of Jerusalem in the Bible.

Gal 4:25 Now Hagar stands for Mount Sinai in Arabia and corresponds to the present city of Jerusalem ...

One is city in Israel. The other one is heavenly Jerusalem.

Gal 4:26 But the Jerusalem that is above is free and she is our mother.

Heavenly Jerusalem is called 'our mother'. Because Heavenly Jerusalem is the figure, presenting God the Mother who is the mother of our souls. In Revelation, Heavenly Jerusalem is described as the wife of the lamb and the bride.

Rev 21:9-10 ..I will show you the bride, the wife of the Lamb. ... and showed me the Holy City, Jerusalem, coming down out of heaven from God.
Rev 19:7 ... For the wedding of the Lamb has come, and his bride has made herself ready

According to the prophecy in Rev 22:17, the bride appears with Holy Spirit and give us the water of life, that is eternal life. Without receiving the Spirit and the Bride, we can not receive salvation.

When we compare the prophecy in Rev 22:17 and Isa 4:3, we can clearly understand that those whoa remain in Jerusalem refers to those who stay in the faith believing God the Mother. So, let's believe in God the Mother and God the Father to the end so that we can receive salvation as those who remain in Heavenly Jerusalem.