Matthew 5:34-35 (NKJV)

³⁴ But I say to you, do not swear at all: neither by heaven, for it is God’s throne;³⁵ nor by the earth, for it is His footstool; nor by Jerusalem, for it is the city of the great King.

Joel 2:32

And it shall come to pass that he who calls upon the name of the Lord shall be saved.

For in Mount Zion and in Jerusalem there shall be deliverance, As the Lord has said, Among the remnant whom the Lord calls.

The God of Abraham, Isaac and Jacob will fight for Jerusalem:

2 Chronicles 20:17

You will not need to fight in this battle.

Position yourselves, stand still and see the salvation of the Lord, who is with you, O Judah and Jerusalem!’ Do not fear or be dismayed; tomorrow go out against them, for the Lord is with you.”

Isaiah 31:4

For thus the Lord has spoken to me: “As a lion roars, And a young lion over his prey (When a multitude of shepherds is summoned against him, He will not be afraid of their voice Nor be disturbed by their noise), So the Lord of hosts will come down To fight for Mount Zion and for its hill.

Isaiah 31:5

Like birds flying about, So will the Lord of hosts defend Jerusalem. Defending, He will also deliver it; Passing over, He will preserve it.

Zechariah 12:8

In that day the Lord will defend the inhabitants of Jerusalem; the one who is feeble among them in that day shall be like David, and the house of David shall be like God, like the Angel of the Lord before them.

Isaiah 28:16-18 (NKJV) A Cornerstone in Zion:

16 Therefore thus says the Lord God: “Behold, I lay in Zion a stone for a foundation, A tried stone, a precious cornerstone, a sure foundation;
Whoever believes will not act hastily.

17 Also I will make justice the measuring line, And righteousness the plummet; The hail will sweep away the refuge of lies,
And the waters will overflow the hiding place.

18 Your covenant with death will be annulled, And your agreement with Sheol will not stand; When the overflowing scourge passes through, Then you will be trampled down by it.

Temporarily King David had a stronghold known as Zion:

1 Chronicles 11:5

But the inhabitants of Jebus said to David, “You shall not come in here!” Nevertheless David took the stronghold of Zion (that is, the City of David).

2 Samuel 5:7

Nevertheless David took the stronghold of Zion (that is, the City of David).

David’s son king Solomon moves the Ark of the Covenant of God to Jerusalem, and from that time Jerusalem became Zion.

1 Kings 8:1

Now Solomon assembled the elders of Israel and all the heads of the tribes, the chief fathers of the children of Israel, to King Solomon in Jerusalem, that they might bring up the ark of the covenant of the Lord from the City of David, which is Zion.