John to the seven churches which are in Asia: Grace be unto you, and peace, from him who is, and who was, and who is to come; and from the seven Spirits who are before his throne;
All Commentaries on Revelation 1:4 Go To Revelation 1
Augustine of Hippo
AD 430
The number seven signifies the perfection of the universal Church, for which reason the Apostle John writes to the seven churches, showing in that way that he writes to the totality of the one Church. City of God 17.4