to be pledged 其主语是人这种用法很少见，好像只出现在圣经中，比如：
Deuteronomy（申命记） 20:7
Has anyone become pledged to a woman and not married her?
Deuteronomy（申命记） 28:30
You will be pledged to be married to a woman
to pledge是简单的发誓。
to be pledged最常用的是承诺捐款或赞助：
One hundred dollars has been pledged to help the school children.
回到你问的这句：
You must be pledged to keeping the secret
我相信这句是Chinglish，不是正常的英语。正常的英语是：
You must pledge to keep the secret.