The Message Dress Code
Legalism is to fall back into rule-keeping - we lose our freedom. Falling into permissiveness means we abuse our freedom.
Gospel freedom is freedom that both takes away the guilt of sin and eats away at the motivation to sin.
The gospel neither leads us to live a guilty life (since God has lovingly accepted us), nor an unholy life (since the God who has accepted us is perfectly holy). To forget the first is to fall into legalism, and lose our freedom; to forget the second is to abuse our freedom. Both mean we lose grasp of the gospel.