Why is the church not mentioned in the book of Revelation after chapter 3 until the end?
The term is absent from Revelation after chapter 3 because the church was not present during the tribulation and had no role in those events. When eternity begins, the church becomes prominent once more and participates in the overall eternal economy. That explains its reappearance at the close.
Les explains that the word "churches" has been absent from the book of Revelation ever since chapter 3. He says the reason is that the church hasn't been around: it was gone all during the episodes of the tribulation, and the church had no part in them. But now that eternity is in view, the church again becomes prominent and is part of the overall eternal economy. This comes near verse 16, where Jesus says He has sent His angel to testify these things in the churches, and then He repeats the I Am.