Legalism doubts the power of the Word. Specifically the Gospel and its power. You see, the fear of Legalism is that the Gospel and it's power of life and salvation. . . just really isn't enough and you have to add some good law and order to make sure that the new life flows.
What follows is a response I wrote on a facebook post asking why, if one thinks that we have no impact upon election why we do any speaking since it's all predetermined.
"The point probably is that it isn't *we* who impact souls, but rather the Word of God that does -- and even if we fail to speak the Word as we ought, God will use others to speak the Word of life to people. Nor is that Word of God made better or stronger or more effective by my own strength or power.
On this we are to be like the Sower of the Seed - we cast the seed, and it simply falls where it wills. We are instruments for God to show His love -- and it's God who is at work through us - it is not I who live but Christ who lives within me. So let us speak because that is who we are.
I didn't see the spot in the document in particular - but the Formula is clear the that Doctrine of Election is to be pure comfort -- in other words, someone who tells you that people are going to hell because "you aren't doing _____" can go stuff it, for God will not loose one of those whom are His - for neither heights nor depths or even my gross incompetence can separate them from God's love.
Well, why do anything? Um . . because we are His workmanship, created in Christ Jesus for good works that we might walk in them. . . or in other words -- because we do, because we are, and when our sinful nature is thrust down by the preaching the Law and the Gospel -- these good works happen. If you are a branch attached the Vine. . . you grow. It's what happens. And you don't grow by focusing on your growth, or trying really hard to. You grow because the Vine gives you growth.
So enjoy the life and love and salvation you have - God is in control. It's all about Him. Enjoy His gifts, delight in His salvation. . . and you know what? He will open your mouth, it will show forth His praise, and that Word will accomplish what He wills -- and I don't need to browbeat you to make it happen."
You see, the bit of assumption in the question was the legalism that comes with a bit of Arminian background we have in the US -- unless there is a direct cause and effect, I can't get a person to act. That's a bit of backdoor, subtle legalism. And it's common. If we don't tell people what they have to do and what good it will cause, they won't. Legalism always assumes that there must be an obvious profit or we won't act.
The thing is -- everything is about the power of God and His Word. And so often we don't realize it.