The doctrine was well established by the 2nd century, but theologians were always puzzled that Paul never mentioned it. In 21st century mainstream Protestant denominations, belief in the Virgin Mary is considered optional.<ref> See for example Donald K. McKim, ''Presbyterian questions, Presbyterian answers: exploring Christian faith'' (2003) [http://books.google.com/books?id=54f0oX23u5EC&pg=PA32&lpg=PA32&dq=presbyterian+%22virgin+birth%22&source=bl&ots=4ACahhqEHy&sig=5fDWvz5HKAHiFC4peLscgZp4UmE&hl=en&ei=fiGYSv-dFJOesgPl_9W1Ag&sa=X&oi=book_result&ct=result&resnum=7#v=onepage&q=&f=false online pp. 31-2 ]</ref> | The doctrine was well established by the 2nd century, but theologians were always puzzled that Paul never mentioned it. In 21st century mainstream Protestant denominations, belief in the Virgin Mary is considered optional.<ref> See for example Donald K. McKim, ''Presbyterian questions, Presbyterian answers: exploring Christian faith'' (2003) [http://books.google.com/books?id=54f0oX23u5EC&pg=PA32&lpg=PA32&dq=presbyterian+%22virgin+birth%22&source=bl&ots=4ACahhqEHy&sig=5fDWvz5HKAHiFC4peLscgZp4UmE&hl=en&ei=fiGYSv-dFJOesgPl_9W1Ag&sa=X&oi=book_result&ct=result&resnum=7#v=onepage&q=&f=false online pp. 31-2 ]</ref> |