1st use: curb. Are you saying that if a non-believer obeys the highway speed limit or stays on the right side of a yellow line, it is NOT useful in serving his neighbor?
"Use" is about how God uses God's Law, not how we use God's Law.
1st use is a curb. But, if the Laws enacted by men to curb behavior fail to serve the neighbor, it is not God using the Law.
2nd use: mirror. Duty under 2nd use - I don't understand what you are driving at? Please explain.
2nd use is a mirror or a rule. It is how God uses the Law to show us our sins. It is the use of the Law which "crushes" us. As it relates to this conversation, it is the use of the Law which which tells us that the priest and the Levite sinned by obeying existing law and ignoring the man in the ditch. It is the use of the Law that tells us it is sinful to tolerate our nation's failure to serve the needs of our disenfranchised population.
Certainly, we are called to support local initiatives and I presume that most/all of the participants in this forum do that. But, the need is greater and there is significant resistance to the idea that government (1st use) has a role to play in addressing that need. I'd suggest that supporting policies and politicians contrary to the best interests of those with the need is failure to love the neighbor and sinful under 2nd use.
Edit: I was taught the 1st and 2nd uses of the Law are for both believers and non-believers. The 3rd use is for believers only. What is your understanding?
That is basically how I was taught and how I believe, except I am not comfortable with the notion of the Law being "for" people. As I noted above, it is how God uses God's Law on people.
This conversation has helped me to better understand the "dispute" about whether there are two or three uses of the Law. Are there really three uses or is "third use" just a way to think about how first and second use function differently in the lives of believers. I have concluded that there is not a lot of substance to that particular disagreement.