The Gemara asks, what is the meaning of the Pasuk, "There is something holy in your midst, and I will not enter the city." The Gemara asks, Does it makes sense that Hash-m would not enter the city because we have holiness in our midst? The Gemara explains that the Pasuk must be preceded with an "UNLESS." I will not enter the Jerusalem of above until I enter the Jerusalem of below. The Pasuk must be read as, unless there is something in your midst, [which would cuase me to enter the earthly Jerusalem], I will not enter the city [the Jerusalem above].
Why does the Gemara have to say that this means there are two Jerusalems? There is absolutely no indication present that there are two Jerusalems! Why can't we explain the Pasuk as, "Unless there is something holy in your midst [in the one and only earthly Jerusalem], I will not enter the city [the earthly Jerusalem]? It actually makes more sense that way. Hash-m is saying I will not enter Jerusalem if you don't make it a place that is worthy of my Shecina.
Why say that there are two Jerusalems?
Avraham Adam Sabzevari, Kew Gardens, NY, USA
The Gemara is not proving from this Derashah that there is a Heavenly Jerusalem. This is apparent from the next line of the Gemara which asks, "And is there indeed a Heavenly Jerusalem?" The Gemara answers that there is, as is apparent from a different Pasuk. This clearly indicates that the first Derashah was only made in this particular fashion once we already know that there are two Jerusalems (which correlate to each other).
All the best,