What has caused the rust? The easy answer: the church lost the gospel. Waves of pragmatism, liberalism, and “Anglo-Catholicism” (a blend of Roman Catholicism and Anglicanism) have swept through the church, leaving wreckage in their wake.
But the actual cause is slightly more subtle. Anglicans still talk about the gospel, a lot. And mission. And even about being evangelical—the new archbishop self-identifies as an evangelical, though he certainly wouldn’t recognize the definition of the term Don Carson and Tim Keller give in TGC’s Gospel-Centered Ministry booklet.
The denomination never lost the words. But it lost the biblical content. In order to keep unity among people who differ over essentials, Anglicanism has increasingly emptied key concepts of their content. So you can sit in a room with 10 Anglican ministers and talk for half an hour about “the gospel” without ever defining the term and always knowing there are probably ten (or eleven!) different views.