Temple of Karnak

Temple of Karnak today remains the worlds largest religious structure, but what is perhaps even more interesting is that it might not have been, or indeed was probably not Egypt's largest temple. Certainly the Temple of Ptah in Memphis, though for the most part completely gone today, may have been larger. It was older, and located in what was often the capital of Egypt, and more often the administrative center of the ancient country. Other temples in the Nile Delta might have been just as large as Karnak, if not larger.
Sites such as Karnak, Dendera and Kom Ombo would most likely fall under the category of "god's mansion". They were more than religious "temples" however. While the god may certainly have been worshiped in these temples, it was also his symbolic home, if not considered his physical residence, and the functions of the temple were as much to serve his or her symbolic physical needs as they were for the god's worship. There was probably little or no "preaching" as such, or carrying the message of the god to the people by priests associated with these "temples". Rather the efforts were directed inward, towards the care of the gods.
Also, though we often make a very specific distention between mortuary temples of kings, for example, and temples such as Karnak, they were actually very similar. Kings were considered gods, and after their death, they required a "mansion" and the same attention as other gods. Both regular and mortuary temples served to keep the name of the king or god alive.