Inattention to sound doctrine is inattention to the gospel. The gospel is doctrine. It is a proclamation about God, our sin, Christ’s saving, substitutionary death and resurrection, and the human need to repent of sin and trust in Christ.
Inattention to the gospel means losing the gospel. Peter says that Satan prowls around like a roaring lion, looking for someone to devour (1 Pet. 5:8). Paul says that Satan takes people captive to do his will so that they would teach false doctrine that corrupts the gospel (2 Tim. 2:24-26). Paul also says that just as Satan deceived Eve, so he strives to deceive us, so that we would believe lies about God (2 Cor. 11:3). Therefore, the gospel will only be preserved by vigilant proclamation and defense, not by lazy inattention.
A church that loses the gospel is no longer a church. If a church loses the gospel, it will cease to proclaim the only message that can reconcile guilty sinners to God. So sinners won’t turn to God in faith. They’ll just think they have. And the church will fill up with self-deceived nominal Christians who blaspheme God’s name by claiming to be his people while they live just like the world (Rom. 2:24). When this happens, a church is dead. It’s no longer a church. It’s a false church, a church that broadcasts lies about God and the gospel, like a military radio station that’s been taken over by the enemy.