1 John 2:19
 They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us. (ESV)
As we read the Apostle John's words we are roughly two millennia removed from the church that he was writing to. He speaks of false teachers that were in the church. They were in one way or another perceived as being a part of the body, and yet they left. In the church two thousand years ago, the members were left with an unsettling feeling because some of their number was now gone. We experience similar feelings today when people leave the church. Let's look at what these people leaving really means.
John lets us know what the people leaving says about them. He says that if they had really been a part of the body, they would not have left. As a result of their leaving it is now obvious what they were really all about. False teaching cannot stay with the faithful teaching of sound doctrine. The two are opposed to one another. In this case the false teachers were not able to establish the stronghold of deception they wanted and so they went out, presumably looking for other places to corrupt.
If you belong to a church you need to consider this passage of Scripture. Sometimes people leave the church and people become concerned. Why did they leave? Not everyone who leaves should be missed! The church that remains does need to evaluate itself. Was the Gospel being preached? Were people being loved? Was sound doctrine being taught? If the answer is yes to these questions and people left it might be a case of "good riddance."
The bottom line is this. The church is supposed to be such a harmonious place of love and truth that no one who was truly following Christ would want to leave it. Be careful to promote sound doctrine and the love of Christ in your church. If some leave maybe it is for the best. Be faithful to the truth. God will build his church.
In what ways are you promoting sound doctrinal teaching in your church? In what ways are you demonstrating the love of Christ? Remain faithful in these things. Christ will build his church.