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.1
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.4
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.