...The classical conception of God includes God's
necessary existence (see Plantinga 1974a, 1974b, 1980; Morris 1987a (in particular chapter 7, Absolute Creation, written with Christopher Menzel), 1987b; Wierenga 1989; Adams 1983; and MacDonald 1991). Perhaps the strongest motivation for thinking that
God exists necessarily is perfect-being or Anselmian theology. On an Anselmian conception of God, God is the greatest possible being; it is in the very nature of God that he essentially (and necessarily) possess all compossible perfections.
Necessary existence is a perfection, it is thought, and therefore God must possess it. One should note that denying God's necessary existence does not entail that God or anyone else can commit deicide. It is far more plausible to think that for any world
W that is such that God exists at some time in
W, God exists at every time in
W. Anselmian theists also typically think that God is essentially (and thus necessarily) omniscient, omnipotent, and maximally good.
A second reason for accepting the necessary existence of God would be available if one took the
ontological argument to be sound. There are, of course, many versions of the ontological argument. But, all of them entail that
God exists necessarily. The most sophisticated version of the argument has been formulated by Alvin Plantinga (1974a, 1974b, 1990). This motivation (the soundness of the ontological argument) is, of course, closely tied to the first motivation (Anselmian theology).
(plato.stanford.edu/entries/god-necessary-being)