Isaiah saw a time of peace for Jerusalem when the people of God would call your walls Salvation and your gates Praise (Isa. Isa. 60:18).
This wall doubtless was a memorial to Gods protection of His people throughout their long history. The New Testament saints would well remember how their lives were hid with Christ in God (Col. Col. 3:1;Col. 3:1). Now, the wall does not need to serve as a means of protection for God Himself dwells in the city. This is further proven by the fact that each of the four walls of the city has three gateways which never will be shut.1with twelve gates
Gates is πυλῶνας [pylōnas] : used Luke 16:20) . . . of the magnificent city gates of Babylon.2 There are three gates on each of the four sides of the city (Rev. Rev. 21:13+). Each gate is of one pearl (Rev. Rev. 21:21+). None of the gates will ever be shut (Rev. Rev. 21:25+) and the glory of the nations and kings of the earth shall be transported through them (Rev. Rev. 21:24+). See Twelve: Jewish Tribes, Completeness.
twelve angels at the gates
Those who do not have the right to the tree of life are unable to pass through the gates into the city (Rev. Rev. 22:14+). The gates may remain open because there is no possibility of the unclean approaching the city. They are eternally relegated to the Lake of Fire which is the outer darkness (Mtt. Mat. 8:12; Mat. 22:13; Mat. 25:30).
twelve tribes of the children of Israel
Children is υἱῶν [huiōn] : sons. Within Scripture, the sons of Israel are the literal, physical offspring of the man Jacob (Rev. Rev. 7:4-8+). Those who deny any distinction between the Church and Israel in Gods purposes are hard-pressed to account for the distinction here, carried into the eternal state, between the twelve tribes of Israel and the twelve apostles of the Lamb (Rev. Rev. 21:14+). Mounce is typical of those who read past the plain distinction in the text and interpret it to mean just the opposite: The juxtaposition of the twelve tribes with the twelve apostles shows the unity of ancient Israel and the NT church. [emphasis added]3 If the Church is the new Israel as Replacement Theology claims, then why the distinction between the names of the tribes of Israel and the names of the apostles of the Churchas two separate groupsmemorialized in eternity? The names of the tribes on the gates and the names of the apostles on the foundations (Rev. Rev. 21:14+) are a memorial to two different peoples of God who occupied different roles in Gods redemptive plan, both of which are found in the eternal city: Israel and the Church.
It is significant that John brings together the twelve tribes of Israel and the twelve apostles here, and makes a distinction between them. Jesus did the same earlier (Mtt. Mat. 19:28; Luke Luke 22:30). This distinction shows the wrongness of identifying the twelve tribes in Rev. Rev. 7:4-8+ with the church.4Even though the twelve apostles were all Jewish, and therefore physical descendants of Jacob, their names are distinguished from the twelve tribes. This reflects the reality that the Church did not exist prior to the Day of Pentecost when the Holy Spirit first began His baptizing work forming the body of Christ (Mtt. Mat. 16:18; Acts Acts 1:5). Although the apostles are all Jewish and will rule over the twelve tribes (Mtt. Mat. 19:28), their greater affinity is as members of the Church. Once more we see the Scriptural declaration that all twelve tribes of Israel are known to God (Rev. Rev. 7:4+). None of the tribes are lost. See Ten Tribes Lost?
2 Frederick William Danker and Walter Bauer, A Greek-English Lexicon of the New Testament and Other Early Christian Literature (Chicago, IL: University of Chicago Press, 2000), 729.