"How could Catholicism not lie at core?"   

Maybe also make a distinction between Catholic faith and Catholic culture when talking about "Catholicism"? 
Catholicism often lies "at core" in different ways, implicitly and explicitly, in different poets like, for example, English poets (all Catholic) -- G. M. Hopkins, John Dryden, Alexander Pope, Siegfried Sassoon, Chaucer, etc. In one context, one could ask, What would the poetry of Hopkins be like without his Catholicism, meaning his faith? But Hopkins, an adult convert, was not as immersed in "Catholic culture" as much as Milosz (or Dante or Chaucer). Yet his poetry is far more intensely religious (with Catholic symbolism) than Milosz's poetry, who was immersed in "Catholic culture." I don't doubt Milosz was a devote Catholic, but he did not write poetry drawing on traditional Catholic iconography or symbolism. His Catholic roots were more implicit in his work than explicit, thus broadening those roots in non-traditional ways.