Gw- I criticise classical theists just as much as I do athiests.
Muro- Because everything in the universe is contingent (doesn't exist in every possible world) and I believe the first cause must be necessary.
To understand why I think that, you need to understand the concept of ontological priority. Ontology is metaphysical subfield which studies being, so ontology refers to being. So for example, the first cause is by definition the most ontologically prior reality, because if something was ontologically prior it that would be the first cause instead. Note that it isn't always the same thing as chronological priority. For example a big debate in metaphysics is whether the essence of a being is OP to its existence, or vice versa, yet these are probably chronologically equal.
Now with necessary properties (that exist in every possible world) there is nothing OP to them because they are the only possibility.
However, with contingent properties, there is something OP to them, which is the various possibilites the property could end up being. There are multiple possibilities of what the property could be, and one way or another of those properties ends up becoming a reality. The problem is that this 'selection process' is ontologically prior (not chronologically prior) to the existence of the actual thing, and of course you can't have something OP to the first cause. Now I obviously don't mean that there was a literal selection process, but you get my point. The first cause can't be a product of some selection process or movement. Even total randomness is still a selection process, just a different methodology. When you think about it, you're saying that the first cause was created by randomness, or some other process, which clearly doesn't make sense.
This is the problem with believing in brute contingencies, because it fails to acknowledge the difference between necessary and contingent properties. BCs basically say that they have the same properties ontologically (existing without explanation), when they don't; a necessary property exists in all possible worlds and a contingent one doesn't.
That was just off the top of my head, I probably didn't do it justice but hopefully I can clear things up if you have any questions.