The Old Testament Jews celebrated the city of Jerusalem because that was where they went to worship. Solomon built the Temple within the walls of Jerusalem on Mt. Moriah. It is believed this was the place where Abraham started to offer Isaac as a sacrifice (Gen. 22:2). Many Christians make trips to Jerusalem to see historical sites and view the places of the crucifixion and garden tomb.
So why is this psalm relevant today? It is because Jesus said His kingdom is not of this world; yet it is in this world but not of it. Because God does not live in a physical city, He lives in believers all over the world. Verse 6 says, “When the Lord registers the nations, He will say, ‘They have all become citizens of Jerusalem’.” Jerusalem is a metaphor for God’s heavenly kingdom as Hebrews 11:16 tells us. “But they (the people of faith just mentioned) were looking for a better place, a heavenly homeland. That is why God is not ashamed to be called their God, for He has prepared a city for them.” And the scripture tells us what John saw in his heavenly vision, “After this I saw a vast crowd, too great to count, from every nation and tribe and people and language, standing in front of the throne and before the Lamb. They were clothed in white robes and held palm branches in their hands. And they were shouting with a great roar, ‘Salvation comes from our God who sits on the throne and from the Lamb’.” (Rev. 7:9-10).
Jesus also told us that where two or three are assembled in My name, I am there with them. That is why we find in Hebrews that we should not forsake (avoid) assembling with others. Each member of the church, the Kingdom of God, has been given a gift, but no one has all the gifts, so we need each other to worship God, to sing His praises, and to study His word. The Gaithers wrote a wonderful song called Get All Excited. When we are together singing and praising God, we can get all excited. That carries over in our personal lives to share with others.