The theist :
(1) God is causa sui (his own cause). He has the reason for his being in himself.
The atheist :
(2) The Universe is causa sui. The Universe has the reason for its being within itself.
Both (1) and (2) state the same problem of inteligibility: conceiving something which is causa sui. The difference is that (1) is a retrograde step in regard to (2), thus being less obvious, because we cannot empiricaly know God. But, the comprehensibility scheme is the same.
Why , in the opinion of the theists, is (1) more reasonable than (2) ?
As I see it (1) is by no means a claim which makes the Universe more inteligible and resolving the common sense difficulty encountered in (2).
But it just violates Ockam's razor by postulating a redundant hypothesis while keeping the comprehensibility problem it tried to solve.