II Timothy 2:14-19
One often thinks the greatest threat to the church is persecution or the wider culture holding the church in contempt. These things can cause some believers to lose confidence and even become ashamed of the gospel. But this is not the greatest threat to the church. Our greatest threat is always from within. False teaching, combined with unholy living is far more deadly than anything the world can do.
So how do we defeat the danger from within? God gives us clear instructions as to what to do.