Pastor John MacArthur answers this important question, in his teaching on Revelation 3:1-6, the church at Sardis:
Death occurs when unbelievers are allowed to take over ministries in the church. It happens when a church becomes more concerned with form and liturgy than it is with life on a spiritual level. Death occurs when a church becomes more concerned about welfare and social ills than it is about salvation. It happens when a church loves systems more than it loves Jesus Christ. And it happens when a church becomes more concerned with material things than spiritual reality. That's how a church dies. It all is a result of sin. . .
What kills a church? Sin--in any form that the church tolerates, whether it is in the members or the leaders. Tolerance of sin begins the cycle; then comes the tolerance of unbelievers in the church until no one cares who is a believer or an unbeliever. The end comes when the man who runs the church isn't a believer. Sins of commission and omission kill a church little by little. When that happens, Christians become carnal. Soon afterwards, unbelievers come into the church, and then a total tolerance for sin exists. The church begins to die, and the people who really love Jesus Christ leave only to be replaced by people who don't know Him. There are many people in our church who can testify to the fact that they have come to a church like ours because they had to leave a church that was dead--it was in the hands of people who didn't love Christ.