Sentence:  Modern man , as Dietrich Bonhoeffer has told us , has `` come of age '' ; and though this process by no means represents an unambiguous gain and is , in fact , marked by the estrangement from the depths that seems to be the cost of human maturation , it is still a positive step forward ; and those of us who so richly benefit from it should <WSD>be</WSD> the last to despise it . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)