However, it's more commonly known as the title of a famous aria from the opera "The Pearl Fishers" by Georges Bizet. In this context, the phrase refers to a sacred place within the temple where the pearl fishers have made a pact of brotherhood.
Therefore, the English translation of "Au Fond du Temple" would be "At the Bottom of the Temple", but when referring to the aria, it's often simply referred to by its title in French.