I was going to respond with the letters to the seven churches in Revelation, but Butterball beat me to it. I'll flesh it out a bit more.
The angels of the seven churches are their messengers.
Revelation 1:20 The mystery of the seven stars which thou sawest in my right hand, and the seven golden candlesticks. The seven stars are the angels of the seven churches: and the seven candlesticks which thou sawest are the seven churches.
Jesus goes on in chapters 2 and 3, to write to the angels of the churches. Jesus rebukes five of the seven.
- Ephesus
- Smyrna
- Pergamos
- Idolatry, holding fast to false doctrine.
- Thyatira
- Jezebel, fornication, eating things sacrificed to idols.
- Sardis
- Remember, hold fast to what you have received and heard. Watch, or I will come as a thief.
- Philadelphia
- Laodicea
- Lukewarmness (neither hot nor cold), poor, blind and naked.
Jesus includes details on how to take corrective action and makes promises if corrective action is taken. He also details consequences for not taking corrective action.
For Laodicea, in particular, Jesus says He will spue them out of His mouth.
The seven churches actually existed in John's day, but they are also types of churches throughout history, from the time of John to present day. The church of today is clearly a type of Laodicea, being lukewarm.
In the letters to the seven churches, we can see how Christians can fall by the wayside and how Jesus will reject them. This isn't the only verse where Jesus says this.
Matthew 7:22-23 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
These verses would indicate that it is possible to be a lost sheep and not return to the flock and be rejected by the shepherd as lost.
The parable of the ten virgins (Matthew 25:1-13) states that the door will shut on those who haven't trimmed their lamps and prepared vessels of oil.