Shared, that is, among Roman Catholics, Anglicans and Lutherans, as well as Methodists and members of the Reformed churches.
Wow. Seriously, people: Wow.
This is the most interesting ecumenical proposal we have heard in years. While challenging, it is achievable; and while it would be a remarkable intellectual effort, it would also be entirely practical.
Of course, catechisms play different roles in different churches, and those roles aren't entirely compatible. Among Lutherans, for example, our two catechisms are actually a doctrinal norm, providing an authoritative reading of Scripture. Among Roman Catholics, the catechism is a handy textbook of doctrines that are normed by external documents, both Biblical and otherwise. Anglicans use theirs "as a point of departure for the teacher," although its presence in the BCP presumably gives it a normative function of some sort.
Still, when you boil it all down, they are teaching tools first, and confessions of faith second. Those are the two natures of a catechism, almost by definition. And what better what to demonstrate the true unity of the Church, which is so often difficult to discern, than by creating a shared teaching tool which lays out the things we hold in common?
It may be argued that the Lima Document already serves this function, and we suppose it does. But it is also full of sections which seek to mediate differences, rather than affirming agreements -- we are thinking, for example, of the necessary even-handedness with regard to antipaedobaptist church bodies.
No, we think that an actual catechism -- a Q&A about the faith, describing what is shared by the great bulk of Western Christianity -- would be not only an immense ecumenical achievement, but also an immensely useful tool for parish education.