I believe the scripture you may be thinking of is Hebrews 12:18, “But you have come to Mount Zion and to the city of the living God, the heavenly Jerusalem, to an innumerable company of angels” (NKJV). The writer to the Hebrews had just been speaking of Mount Sinai and the terrible events that accompanied the giving of the Ten Commandments. Let’s read verses 18-21, “For you have NOT COME to the mountain that may be touched and that burned with fire, and to blackness and darkness and tempest, and the sound of a trumpet and the voice of words, so that those who heard it begged that the word should not be spoken to them anymore. (For they could not endure what was commanded: ‘And if so much as a beast touches the mountain, it shall be stoned or shot with an arrow.’ And so terrifying was the sight that Moses said, ‘I am exceedingly afraid and trembling’).” This was written to Jews who had professed faith in the Lord Jesus Christ yet they were still clinging to the Law of Moses and were in danger of trusting in the keeping of the Ten Commandments for salvation. So, they are reminded in this passage of the frightful experiences on the day the Law was given from Mount Sinai. One must ask, “Why did God cause these terrifying disturbances in nature to occur when the Law was given?” Ah, because the Law revealed God’s intrinsic holiness and intolerance for sin, and that He must judge those who are guilty of breaking the Law.

In verse 18 we have a blessed contrast, for the believer in Jesus Christ is not “under the Law,” but “under grace” and that is what Mount Zion represents. In John 1:17 we read, “For the law was given through Moses, but GRACE and truth came through Jesus Christ.” The one who puts his trust in Jesus Christ for his soul’s salvation is blessed on the principle of GRACE. Grace is the opposite of Law, for the Law demanded righteousness from sinners that they could never obtain, but grace imputes to the sinner a righteousness he could never earn. It is an absolute FREE GIFT from God, though it cost Him the life of His Son to offer it to sinners. “For by GRACE are you saved through faith, and that not of yourselves; it is the gift of God” (Ephesians 2:8). “Now to him who works, the wages are not counted as grace but as debt. But to him who does not work but believes on Him who justifies the ungodly, his faith is accounted for righteousness” (Romans 4:4-5). One has said, “Law has its Mount Sinai but faith has its Mount Zion. This heavenly mountain symbolizes the combined blessings of grace—all that is ours through the redeeming work of Christ Jesus” (William MacDonald). So, even though there was a LITERAL Mount Zion on the north side of Jerusalem (see Psalm 48:1-2), it has become a symbol representing God’s grace and all the blessings we (believers in Christ) have through our Lord Jesus Christ.

As to the “New Jerusalem,” our verse also says we have come to “the city of the living God, the heavenly Jerusalem.” This is referred to in Hebrews 11:10 where we learn that Abraham “waited for the city which has foundations, whose builder and maker is God.” It is described a great detail in Revelation chapters 21 and 22 (which I would encourage you to read). It is a LITERAL city in heaven where believers from Old Testament times and believers who make up the church will live. This is taught in verse 23, “to the general assembly and church of the firstborn who are registered in heaven…to the spirits of just men made perfect.” Scripture teaches us that Jerusalem on earth will be the center of God’s government during Christ’s Millennial Kingdom. The New Jerusalem will be God’s “heavenly capital” for all eternity. (278.5) (DO)