Sentence:  He says : `` Beside the Protestant philosophy of Progress , as expressed in radical or conservative millenarianism , should be placed the doctrine of the democratic faith which affirmed it to be the <WSD>duty</WSD> of the destiny of the United States to assist in the creation of a better world by keeping lighted the beacon of democracy '' . 
Meaning of the word marked with <WSD> tags:  work that you are obliged to perform for moral or legal reasons