First thing you need to do is acquire 500 gold which is the price required to become "Defender of the faith", unless someone else already has claimed the title go to "Religion" tab and press the "defender of the faith" Icon which is available for most major religion such as Catholic and Sunni. If someone has claimed it wait for that ruler to die and claim the title before another ruler claims it.
This achievement should unlock when you payed the price and claimed the title.