In terms of the Ontological Argument’s first premise, which is, “It is possible that a Maximally Great Being (God) exists,” there is a particular justification/warrant that the theist can appeal to in support of the premise. One of the ways is to appeal to the other arguments of natural theology (arguments for God’s existence) to raise our confidence in support of thinking that it is possible that a Maximally Great Being exists.
My problem with this is that I think an atheist can do the same thing in terms of us getting to doubt the first premise. Consequently, the atheist can appeal to incompatibility arguments to render God’s existence not even metaphysically possible, and they can appeal to the evidential problem of evil to raise serious doubt on whether premise 1 of the Ontological Argument is true. In fact, the atheist could plug the conclusion from the evidential argument (God doesn’t exist) into the ontological argument to conclude now that, necessarily, there is no God. It might be objected by the theist that the atheist must establish that her arguments are good/strong; however, the atheist can respond in the same manner in regards to the theist using his arguments to support the ontological argument.
What this seems to mean is that before we can establish that God’s existence is possible or impossible, we have to first establish whether God exists or not from other arguments. Or it means that we first need to establish whether God’s existence is possible or not from other arguments.
This either renders the ontological argument almost useless on its own or question begging at worst. It would almost seem useless on its own because it would be of far less importance. Why of lesser importance? Because we have to look at the other arguments first, and so the ontological argument would stand or fall on whether those arguments succeed; this is of course granting that appealing to the other arguments in support of the ontological argument’s first premise is the only way to skip the scales in favor of the ontological argument itself. And why might it be question begging? Because the whole debate is about whether the arguments for God’s existence even succeed or not. For all we know, they do; however, without further argument and support of the arguments, we haven’t established that God exists, which means that we haven’t established that it’s metaphysically possible that God exists.
So, it seems we should actually see whether the arguments for and against God are sound/weak.