keystone
n
1 (architecture 建) central stone at the top of an arch locking the others into position 拱顶石.
2 (usu sing 通常作单数) (fig 比喻) most important part of a plan, an argument, etc on which all the other parts depend （计划、 论据等的）基础, 主旨
Belief in a life after death is the keystone of her religious faith. 她的宗教信仰的基础是相信死後有来生. Cf 参看 corner-stone (corner1).