Renaissance sacred music had become somewhat common to encounter Renaissance composers incorporating secular songs/melodies into sacred music compositions. The Church had an opinion on this, but what

Renaissance sacred music had become somewhat common to encounter Renaissance composers incorporating secular songs/melodies into sacred music compositions.  The Church had an opinion on this, but what are your thoughts? Is there always a clear distinction between sacred and secular music? What makes a piece of music one or the other? Can secular melodies or musical styles be successfully integrated into sacred music, and is it appropriate to have secular styles or melodies in sacred music (past or present)? Why or why not?