Would you like to see some data for yourself?  Click on the first link on the right and scroll down the page.  Included in the list are studies from nonreligious organizations like W.H.O.   Also a great study is listed which included data from by Calcutta, India... that would be Blessed Mother Teresa's teaching!

Lastly, don't forget to check out the other links.  They have some great explanations about Method-related effectiveness and User-related effectiveness. 

Lastly, don't be afraid to read the package insert for oral contraceptives (a.k.a. The Pill).  Those package inserts will even admit that OM and STM methods of NFP have 97-99% effectiveness.