Joel - one popular interpretation is that these individuals were never believers to begin with. They profited by being within the body, but spiritually they were reprobate. Remember what Paul wrote:
A Jew could profit by being under the economy of the Law while not having faith. The New Testament church operates in a similar fashion. A person could benefit from the love of the body and become
partakers while still being lost. But at what cost? Without faith they sear their conscience to the point where their sin becomes inconsolable, even to the power of the gospel.