Platonism, as I’m using the term, is the view that there are two distinct and radically different realms of reality. The first (lower) realm is material, changeable, transient, and accessible via our senses; it is what we usually call the natural or physical universe. The second (higher) realm is immaterial, unchangeable, eternal, and not accessible via our senses. (Platonism is named after the ancient Greek philosopher Plato, who defended something like this two-level view of reality.)
According to Platonism, what we call “God” is essentially the highest principle of truth, goodness, and beauty in the transcendent realm, and things in the material realm are true, good, and beautiful to the extent they conform to God. For the Platonist, God is the highest of all things, perfect in every respect and utterly transcendent. God is quite distinct from the universe; the universe isn’t within God. However, God isn’t a personal being. God is more like an impersonal divine principle or abstract ideal.
Platonism holds a significant advantage over other worldviews (such as Atheism, Panentheism, and Pantheism) because it posits an absolute, objective standard of goodness that is distinct from the universe. The universe is a mixture of good and evil (as the newspapers confirm for us every day), but God is not. So on the face of it, Platonism gives a plausible explanation as to why there are real, objective distinctions between good and evil, truth and falsity, and so forth. There is an ultimate standard of truth, goodness, and beauty—namely, God—and things in our universe (including us) are good or bad to the extent that they conform to this standard.
However, Platonism faces a host of questions that turn out to be rather tough to answer. Why does the material universe exist in the first place? How are these two radically different realms connected? How can one influence the other?
According to most Theists, God is a personal being with intellect, intentions, free will, and causal powers. God has thoughts and plans, he makes free choices, and he has the power to influence other things—to create them, sustain them, change them, direct them, and so forth. The problem for Platonism is that an impersonal principle or abstract ideal doesn’t have any of these capacities or powers. So the Platonist can’t readily explain why an orderly material universe exists at all, why it bears the marks of intelligent design and contains personal beings like us, and why human life has meaning, purpose, and moral direction. These specific features of our universe are far less surprising if God is a powerful personal being.
On a more practical level, why should we believe for a moment that Platonism’s God cares in the slightest about what happens in our universe and in our lives? (Can an impersonal, transcendent principle of goodness “care” about anything?) If we’re irrelevant to God, why should God be relevant to us?
Platonism may have its virtues, but many people find it less philosophically and religiously satisfying than a Theist worldview in which God is understood to be a personal Supreme Being.
(from James Anderson's "What is Your Worldview?")