Sentence:  In this connection , it has been observed that the increasing number of Irish Catholics , priests and laity , in England , while certainly seen as good for Catholicism , is nevertheless a <WSD>source</WSD> of embarrassment for some of the more nationalistic English Catholics , especially when these Irishmen offer to remind their Christian brethren of this good . 
Meaning of the word marked with <WSD> tags:  the place where something begins, where it springs into being