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 <WSD>be</WSD> the duty 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:  have the quality of being; (copula, used with an adjective or a predicate noun)