Descartes thought it was, although his reasoning was circular.
Personally i think it is, although at the same time i do not regard this as proof that god exists, at any rate not as something in the external world.
But it is interesting that everyone can have some notion of a god, and then so seldom do people wonder if their particular notion has much to do with that of the next person. It would seem to me that it is highly likely that each one has something very different, and thus religious discussions re based on a wrong standing point, that accroding to which the members of the discussion actually refer to the same, or at least an analogous, thing.