Pope Saint John Paul II's apostolic exhortation titled, Christifideles Laici, discusses how we are collaborators with God through our vocations. Follow this link and read the section starting at 61.

http://w2.vatican.va/content/john-paul-ii/en/apost_exhortations/documents/hf_jp-ii_exh_30121988_christifideles-laici.html