Deuteronomy（申命記） 20:7
Has anyone become pledged to a woman and not married her?
Deuteronomy（申命記） 28:30
You will be pledged to be married to a woman
to pledge是簡單的發誓。
to be pledged最常用的是承諾捐款或贊助：
One hundred dollars has been pledged to help the school children.
回到妳問的這句：
You must be pledged to keeping the secret
我相信這句是Chinglish，不是正常的英語。正常的英語是：
You must pledge to keep the secret.