The oldest Mission in California- established in 1769 by Junipero Sera- before the US was even a country. It has had several reincarnations since then and the present church was built around 1865. It was declared a Basilica in 1976 by Pope Paul VI and is still an active Catholic church. You can find more of its history here: http://www.missionsandiego.com/mission_history.htm
It is located just east of Qualcom stadium at 10818 San Diego Mission Road.
Inside the church.