Sentence:  The `` belaboring '' <WSD>is</WSD> of course jocular , yet James was not lacking in fundamental seriousness - unless we measure him by that ultimate seriousness of the great religious leader or thinker who stakes all on his vision of God . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)