Some simple equations:
Subsidiarity + Solidarity = Democracy
Subsidiarity – Solidarity = Tyranny
It really is that stark and that simple. The good news — at least for those of us who don't favor tyranny — is that these two things are not easily separable. Subsidiarity clarifies solidarity, but it cannot exist apart from it. And solidarity without the focus provided by subsidiarity is too amorphous and overwhelming to provide useful guidance.
Both principles share the same premise: Everyone is responsible for everyone else. You are your brother's keeper and your sister's keeper and everyone, everywhere, is your brother and sister. No exceptions. No exclusions. No excuses. Moral obligation and mutual responsibility is universal and boundless.
I realize that some of my fellow evangelicals can be put off by the Catholic terminology here, but for those evangelicals seeking a more explicitly biblical basis for this universal and boundless responsibility, let me just point to our favorite Bible verse, John 3:16. "For God so loved the world." The word "world" there is cosmos — meaning the whole thing entire (boomdeyada, boomdeyada).
So this is the starting point, the foundation. Everything else that solidarity and subsidiarity means builds upon this. I am responsible for you and you are responsible for me and we both, you and I, are responsible for everyone else, without exception, just as everyone else, without exception, is responsible for you and for me and for one another.
Priority, but not limits. There are no limits. Subsidiarity tells us where to start. It does not tell us where to stop. Or that we can stop, ever. It won't allow us to stop. The purpose of subsidiarity is to help each of us determine where our own particular responsibilities begin, but it does not tell us where those responsibilities end because they never do.
Subsidiarity is an expression of solidarity, not an exemption from it.
Those who would treat it as such an exemption imagine that diminishing solidarity somehow enhances freedom. They're wrong about that. It works the other way 'round. We'll explore why later, but here I just want to make this one point as clearly and forcefully as I can: Subsidiarity tells us where our responsibilities begin, not where they end.