STT,
Others are making good points especially in regards to philosophy. I am an amateur in philosophy and only know some basics here and there. But here are some of my thoughts relating to your question that may help:
(1) It is hard to imagine realities such as God or spirit or angels, or “being without extension and form,” as you say. But that does not mean they are unreasonble. In fact, it would seem at least one reality – the supreme reality – would have to be without *material *form (your definition), or else it could not be the ultimate reality. If this being indeed had form, then it would require explanation as to why it has this form and not that form, for example.
(2) We imagine using our experiences and senses, which inform our understanding of the world around us. This makes it hard to “understand” in the sense of visualizing those realities that exist beyond everyday experience. One idea I am sure you would agree with is the idea that some reality (even if you say the Universe) must have never failed to exist – whether you say this means infinite time, or eternity without time. Either way, this is VERY hard for humans to imagine and understand. How could something have “always” existed? Nevertheless, we know this has to be. For, after all, nothing at the moment would or could exist if at one point there was, in fact, nothing.
(3) Another point: Don’t forget the materialistic bias. The complication you bring up revolves around the idea that intellectual processes are ultimately physical or biological. I will not be able to argue for the spirituality of the soul or mind at length here. But I recommend Robert Spitzer, S.J., books. They are easy to read and understand. One point, which may not be extremely helpful for you but was nevertheless interesting to, was his summary of research studies on near death experiences whereby persons who are clinically dead (brain dead, etc.) later are revived and report things such as heavenly visitations, consciousness survival. Even blind people report SEEING for the first time during this brief state of clinical death.