The question isnt really why should this article cite that he was a priest moreso why shouldnt it This is an encyclopaedia and I think that the fact that he is a priest who supports the big bang theory is quite an interesting fact I can completely envision a scenario where someone notes that fact within this article follows the link to Lemaitres article and finds out more That scenario would not happen if the fact wasnt mentioned and if the reader was a person unaware of the meaning to the term Monsignor that scenario would not happen if explicit mention of the fact wasnt included