I read this question and answer, from Paul Tripp on the Desiring God blog. For those of you not familiar with Paul, he's the president of Paul Tripp Ministries, whose purpose is "to place the truths of the Gospel—the person and work of Jesus Christ—in the middle of the messy and confusing situations and relationships in which every person lives." He's also on the pastoral staff of Tenth Presbyterian Church in Philadelphia and is a professor at the Redeemer Seminary in Dallas, Texas.
You've said that we deny the gospel when we use guilt, threats, or manipulation to motivate good behavior. How is that different from the ways God motivates us?
(The following is Paul's answer, edited and slightly abridged.)
God's warnings and encouragements are not just tools to manipulate my behavior, because it's clear that God is not satisfied with that. I think that what he is after is my heart.
Isn't that exactly why God rages through the prophets against Israel? He says, "These people honor me with their lips, but their hearts are far from me. I won't take that. I don't want your holy days. I don't want your solemn assemblies. I don't want that stuff! I want your heart."
That's not what many parents want. If a parent is yelling at a child, it's not because they want their hearts. They want to create enough fear in that child so that they'll do what they want them to.
If I had the heart in view, I would never motivate that way because it's damaging to the heart of a child. God's warnings, on the other hand, are never damaging to the heart. They're after the heart, because he knows if he doesn't have my heart, he doesn't have me.
In the Old Testament, we see that God's people would actually, on the way to the temple, make sacrifices to Baal. On the way to the temple! That's how deep the idolatry was in their heart. If God is zealous after our hearts, he's never going to allow that.
But often a parent will tell a child to do something, and the child will yell at the parent as he is going down the hallway. But as long as the kid does what he was told to do, the parent is satisfied.
Now what is the yelling telling me as a parent? It's telling me I don't have a submissive child. I don't have that child's heart. That's actually disobedience dressed up as obedience, because the child is raging against authority as they're technically doing what they've been asked. God would never call that obedience.
So when parents call that obedience, they're being satisfied with something God wouldn't be satisfied with. They're telling their children something that is very dangerous, letting them think that they can be obedient without an obedient heart.
This happens when I'm focused on doing whatever I can do to get my kid to do what I want him to. But if I could get at his heart for a moment, then when he goes to bed with his room still messy, it's still a victory. Maybe we haven't gotten to the room cleaning yet, but we've gained ground in the most important thing—the Lord ruling his heart.
For more on parenting from Paul, see this Q&A video from Desiring God.