It's a response

First, obedience is a response to what God has already done, while legalism is doing right to gain a reward. Obedience recognizes that God’s grace has already been given in the past, and, in the present, God continues to will the salvation of people from oppression. This basic will of God for history is a given fact and cannot be earned by any deeds on our part. What we do is a response to this given; we enter into action on this basis as co-workers with God. Thus, obedience understands law in the context of grace and history; it understands biblical law as a liberation instrument which, as we will see, is to realize, maintain, and promote shalom. Legalism, on the other hand, believes we must do what is right in order to earn God’s favor or love. If we are good enough then God will reward us. Here, rather than responding to the divine will, we are working for wages.— Shalom, page 74

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Excellent analysis, but I was disappointed by the lack of mention of the Holy Spirit. As I mentioned in the first excerpt, that's the thing that always strikes me about so many Mennonite books. They get it right, but neglect the importance of the Holy Spirit in making it happen...
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