God for the variety of existence constitutes His immunity to nonexistence.
However, Spinoza is right in saying that a thing can only fail to exist if something 'prevents' it from existing; in other words, all facts are partly positive. But he misconstrues this to mean that effects are necessitated by their causes. They are necessarily prevented from existing if the causes are sufficiently unfavorable, but even favorable causes cannot reduce to zero the creativity--involving contingency--which is becoming itself. One must also take temporal aspects into account in considering how one thing may prevent another from existing. Granted that creativity could, at a certain point, have taken another course than it has, the course actually taken henceforth excludes this other possibility. It was, but no longer is, an 'open possibility'. But no possibility is closed unless the realization of some incompatible possibility has closed it. Only the future is still an open possibility, and an eternal being, which can never be merely future, can never be an open possibility. Whatever made its nonexistence a fact would also make its existence impossible.
God's absolute "power to exist" is His ability to assimilate any and every causal condition, to make it 'favorable' to some appropriate responsive state of His own awareness. This is the opposite of being influenced by nothing other than Himself. Nothing can be merely other or alien to God; all have something 'in common' with Him. This neoclassical view is about equally far from that of Spinoza and that of Anselm.
Here is one of the few writers who have arrived at a good