If you’re a theist, you probably already accept that God is vividly aware of everything that is the case. The chocolate cake in your fridge; the birds chirping outside; the neurons firing in your brain, and what thoughts and feelings this generates within your mind: He sees (and hears and tastes and smells and feels) it all.
You probably also believe that events unfold according to God’s will. When laws of nature govern how one moment develops into the next, that’s because God willed those particular laws rather than some alternative. You may further hold that God sustains all of reality. Without Him, it would pop right out of existence. Nothing exists truly independently of God, on a very natural way of thinking (for theists).
If you accept all that, there’s a decent chance that you’re already an idealist, and you just didn’t realize it. Alternatively, if you previously rejected the final step—and assumed that God created a self-sustaining material reality external to his mind—then I think there are good reasons for you to reconsider.