"There are some conclusions which the Holy Spirit would mark indelibly upon our minds. One of these is the fact that the mount Zion of John’s vision is not the literal, physical, earthly mount Zion. Look to the Middle East, to the mount Zion in the earthly Jerusalem. Do we see the glorified Lamb of God standing there? Has He ever been seen standing there at any time during the past two thousand years? Absolutely not! Nor is the Lamb of God going to stand visibly again in flesh on that earthly mount Zion. Why should He? “For ye are not come unto the mount that might be touched…but ye are come untomount Zion…the heavenly Jerusalem” (Heb. 12:18,22). Jesus Christ Himself is the chief cornerstone in the spiritual house of God raised up upon the heavenly mount Zion! “Wherefore also it is contained in the scripture, Behold, I lay IN ZION a chief corner stone, elect, precious: and he that believeth on Him shall not be confounded” (I Pet. 2:6). And again, “Why do the heathen rage, and the people imagine a vain thing? The kings of the earth set themselves, and the rulers take counsel together against the Lord, and against His Christ. For of a truth against Thy holy child Jesus, whom Thou hast anointed, both Herod, and Pontius Pilate, with the Gentiles, and the people of Israel, were gathered together. For to do whatsoever Thy hand and Thy counsel determined before to be done. He that sitteth in the heavens shall laugh: the Lord shall have them in derision. Yet have I set my king upon my holy hill of Zion. I declare the decree: Thou art my Son, this day have I begotten Thee” (Ps. 2:1-7; Acts 4:25-28). To this heavenly and spiritual mount Zion come the 144,000 who “follow the Lamb whithersoever He goeth.” Can we not see by this that the mount Zion that John saw in his vision was a “sign.” It signified, symbolized, the invisible heavenly mount Zion; and that is where the firstborn Son of God has been laid as the chief cornerstone! And it is there upon the heavenly, spiritual mount Zion that the 144,000 overcomers follow the Lamb!"

This is a great study on spiritual Zion:

https://www.godfire.net/eby/rev-159.html