In the first chapter of Revelation, the glorified Christ appears to John to give him messages for the seven churches of Asia. In addition to the description of his shining appearance, John sees Jesus standing among seven golden lampstands, holding seven stars in his right hand. Jesus tells John what these symbols represent.
As for the mystery of the seven stars that you saw in my right hand, and the seven golden lampstands, the seven stars are the angels of the seven churches, and the seven lampstands are the seven churches. (Rev. 1:20)
As Jesus then gives a message to each of the seven churches in Revelation 2-3, he begins each address with the words "To the angel of the church in ________ write", with the name of the church filling in the blank. This scene and these words raise the question: Who are these angels, and why does Jesus address the letters for the churches to them?
According to G.K. Beale in his commentary, four different interpretations are common. The angels are seen as either 1) heavenly beings in general, 2) heavenly beings who are representatives of or guardians over these seven churches, 3) human leaders or representatives of the churches, such as the pastor of the congregation, or 4) personifications of the prevailing spirit or character of the churches. To determine which is the proper interpretation, note that the angels are represented as stars.
In the Bible, stars and angels often correspond to rule and government. The stars are placed in the sky to govern the night (Gen. 1:16-18); Ps. 136:9). In Job 1:6 angels (called the “sons of God”) come before God in his courtroom, and in Job 38:7 the angels are viewed as morning stars praising God over his creation. Angels carry out the rule of the Lord on earth (Ps. 78:49; 91:11; 103:20; 104:4). What John is seeing in his vision is that Jesus, the ascended King who rules over all things, has angels there before him in his heavenly courtroom as ministers carrying out his rule. Even today, it is common for governments to use stars to represent rule. Think of how the flag of the United States uses 50 stars for that purpose.
More specifically, as the angels of the churches, these local congregations in the cities of Ephesus, Smyrna, Pergamum, Thyatira, Sardis, Philadelphia, and Laodicea have these angels as Christ's hand-selected representatives for them standing before him in heaven. They are representing these churches' interest before the Great King.
Later on in Revelation, when God pronounces judgment on the enemies of the church, it is these seven angels who carry out the execution of the punishment. What God is telling each church through this symbolism is that her interests are known by Him, and that her heavenly representative shall take care of her. Perhaps our view of "guardian angels" needs to be adjusted more from the individual to the corporate. As the psalmist says, "The angel of the Lord encamps around those who fear him, and delivers them" (Ps. 34:7). So of the four possible interpretations listed above, the second one seems most fitting.
That answers who the angels are. Now why are each of the letters addressed to “the angel of the church” of the given location (Revelation 2:1,8,12,18; 3:1,7,14)? Again, some note that we get our word for “angel” from the Greek word “angelos,” a word that can also mean "messenger." They then surmise that the messenger of a church is the preacher, and so think he is the one meant, such as in option 3) above. Yet because, as Beale points out, all the many other times (over sixty!) in Revelation that the word angel is used it refers to a heavenly being, it is a bit inconsistent to force the idea of an earthly messenger upon it.
Rather, we should recall that the heavenly angels are God's heavenly messengers to us. Angels are often seen in the Scriptures as those who transmit the word of God to His people. For instance, we are told that the written law was given to Moses by an angel (Acts 7:53; Galatians 3:19; Hebrews 2:2). Surely what is being communicated to these churches is that Jesus knows each congregation of them specifically. Thus, the unique, direct message that Jesus is speaking to each congregation is being written down, given to their representative angel to communicate to them through John, and then backed up by their authority.
Wow - how vital it is for the church to hear and heed that message! To the congregations who listen and overcome, their names will be confessed by Jesus before His Father and their angels (Rev. 3:5). But to those who fail to do so, their lampstand will be removed (Rev. 2:5). In other words, to the church the ultimate message is either shine like a light, like a star, like an angel in this dark world, or know you ultimately will be destroyed by one. That sure adds a whole new dimension, indeed a heavenly one, to our gathering each Lord's Day.