What is the role and significance of Zion in Christianity?

Question

The meaning of Zion is uncertain. There is a hymn related to Zion. The refrain part says,

We’re marching to Zion,
Beautiful, beautiful Zion;
We’re marching upward to Zion,
The beautiful city of God.

What is the role and importance of Zion in Christianity?

Answer 1

As the Wikipedia article on Zion says, Zion is a synonym for Jerusalem.

Revelation 21 depicts the New Jerusalem, the dwelling-place of God with men:

^21:2And I saw the holy city, new Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband; ³ and I heard a loud voice from the throne saying, “Behold, the dwelling of God is with men. He will dwell with them, and they shall be his people, and God himself will be with them. ...” [RSV(CE)]

It is this Promised Land [a similar metaphor] to which Isaac Watts’ hymn refers:

The hill of Zion yields
A thousand sacred sweets
Before we reach the heav’nly fields,
Before we reach the heav’nly fields,
Or walk the golden streets,
Or walk the golden streets.

Refrain

Then let our songs abound,
And every tear be dry;
We’re marching through Immanuel’s ground,
We’re marching through Immanuel’s ground,
To fairer worlds on high,
To fairer worlds on high.

Refrain

There’s a Wikipedia article on the New Jerusalem as well.

Based on the Book of Revelation, premillennialism holds that, following the end times and the second creation of heaven and earth (see The New Earth), the New Jerusalem will be the earthly location where all true believers will spend eternity with God. The New Jerusalem is not limited to eschatology, however. Many Christians view the New Jerusalem as a current reality, that the New Jerusalem is the consummation of the Body of Christ, the Church and that Christians already take part in membership of both the heavenly Jerusalem and the earthly Church in a kind of dual citizenship. In this way, the New Jerusalem represents to Christians the final and everlasting reconciliation of God and His chosen people, "the end of the Christian pilgrimage." As such, the New Jerusalem is a conception of Heaven.

Answer 2

In order to explain Zion in Christianity let me go to the origin of Zion. The Jebusites owned the upper hills of Jerusalem. Judeans and Benjaminites owned the lower parts of the hills. The upper was enemy strongholds, hence David captured the Jebusite stronghold in 1005 BC (See map of Jerusalem ). David gave it the name Zion after it became his residence & capital (2 Samuel 5:9; 1 Chronicles 11:5-7). Zion's original meaning was “dry or arid mountain or hill.”

Zion became the city of David (2 Sam 5:7, 9; 6:12, 16; 1 Kin 8:1; 1 Chr 11:5, 7; 15:1, 29; 2 Chr 5:2). Zion became synonymous with Jerusalem’s name (Psa 87:2, 5; 149:2; SoS 3:11; Isa 33:14, 20; Jer 9:19; 30:17; Zec 9:13.)

David gave importance to Zion by placing the Ark of the Covenant in Zion (2 Sam 6:12,16). There was worship in Zion (2 Kin 19:21, 31; Psa 9:11; 48:2, 11, 12; 74:2; 132:13; 137:1; Isa 40:9; Jer 31:6; 50:5; Joel 2:1, 15). God welcomed the worshippers in Zion as His own people (Isa 51:16). God was present in Zion (Isa 52:7-8). People will bow down to God in Zion (Isa 60:14). God offered salvation to the worshippers at Zion (Isa 62:1, 11). Worshippers sang songs and played music in Zion (Psa 137:3; Joel 2:1, 15; Isa 35:10). Zion is God’s city (Psa 87:2, 3; Isa 60:14). Solomon brought the Ark of the Covenant from Zion to dedicate the Temple and didn't return the Ark to Zion (2 Chr 5:2; 2 Kin 8:1). God promised the restoration of Zion (Isa 51:3, 11, 16; 52:1, 2, 7, 8; 59:20; 60:14; Oba 1:17-21; Zep 3:14, 16; Zec 1:14, 17; 2:7, 10; 8:2, 3; 9:9, 13).

Fast forward to the New Testament writings. Zion is the city of Jesus Christ in Jerusalem in the Millennial reign (Heb 12:22; Rev 14:1). Angels and all the redeemed people will be there. Jesus Christ will rule from Zion. Isaiah prophesied this in Isa 2:2-4.

Hebrews 12:22

But you have come to Mount Zion and to the city of the living God, the heavenly Jerusalem, to an innumerable company of angels.

Isaiah 2:2-4

² Now it shall come to pass in the latter days that the mountain of the LORD's house shall be established on the top of the mountains and shall be exalted above the hills; and all nations shall flow to it. ³ Many people shall come and say, "Come, and let us go up to the mountain of the LORD, To the house of the God of Jacob; ⁴ He will teach us His ways and we shall walk in His paths." For out of Zion shall go forth the law, And the word of the LORD from Jerusalem.

Revelation 14:1

Then I looked, and behold, a Lamb standing on Mount Zion.