People will leave your church. The majority of your core group (typically 80%) will not be with you in 2 years. This is just how it is! As leaders, we need to understand....even expect this to happen. Steve Sjogren calles these people "scaffolding" in one of his books - Community of Kindness. It seems as God sends people in for a season in order to help get things going. Then for some reason or another, they leave.
There are many reasons for a person to leave. Some are legitimate such as a calling to another area, or a return to a sending church. Some reasons are selfish in nature. Some people will stay as long as the church is small. Once they see growth, they bail. Some will go quietly and others will go crazy and try and bring others out. Some will talk to you about their leaving....some people will leave and then talk about you : )
If you are like me....you might take it personally and wonder what you may have done or said to make them want to leave. But I believe we should expect this to happen and when people leave, we should bless their departure and thank them for their contribution to the church plant. Dont beat yourself up with doubts and dont try and keep someone who is determined to leave. By all means, NEVER promote someone to leadership in order for them to stay. This will only lead to problems down the road. Take the high road and bless them and get on with the ministry that God has called you.