God is traditionally described as causa sui, as self-caused. Construed positively, however, the notion appears incoherent. Nothing can function as a cause unless it exists. So if God causes his own existence, then his existence as cause is logically prior to his existence as effect. God must 'already' (logically speaking) exist if he is to cause himself to exist — which teeters on the brink of incoherence if it does not fall over.
So I suggest that causa sui be read privatively rather than positively, as affirming, not that God causes himself, but that God is not caused by another. This reading may gain in credibility if we look at some similar constructions.