I think the title of this post is a useful way to frame the issue. Cambridge philosopher G. E. Moore once attempted to prove the existence of an external world by saying:
How? By holding up my two hands, and saying, as I make a certain gesture with the right hand, ‘Here is one hand’, and adding, as I make a certain gesture with the left, ‘and here is another’
Now, some people might find it preposterous to say God's existence is more certain than whether my hands are real, but that's deceptively simplistic. To begin with, we need to distinguish between certainty and certitude. By "certitude", I mean a psychological state of absolute conviction or confidence. Let's say that I have greater certitude in the reality of my hands than I have in the reality of God.
Now, let's compare that to certainty in the sense of a logical or mathematical rigor. A mathematical proof might be rock solid. The gold standard of proof. Demonstrably true.
But even though a mathematical proof is more certain than the existence of my hands, I might have greater certitude in the existence of my hands. For one thing, some mathematical proofs are fiendishly complex, and there's the nagging doubt that it may suffer from some subtile but undetectable fallacy.
And yet, certitude in the existence of my hands can be misplaced. Moreover, this doesn't require outlandish thought-experiments. When I dream, I dream that I have a body. In the dream, I'm sure I've got a pair of hands. But my hands are imaginary in the dream.
Likewise, someone who's psychotic may be utterly convinced that his hallucinations are real. Take the famous title of a book by Oliver Sacks: The Man Who Mistook His Wife for a Hat. So, if you think about it, the notion that your hands might be illusory isn't that far-fetched.
Another way to approach the issue is to ask how much hangs on the denial of each alternative. What must the world be like if your hands don't exist? What must the world be like if God doesn't exist?
In the case of illusory hands, you don't necessarily have to make any adjustments to a common sense view of the world. I just gave two examples. Dreaming and psychosis are consistent with a physical world. External objects.
There is, of course, a more radical interpretation. And that is idealism or virtual reality. Where what we take to be the external world is a computer simulation, a la The Matrix. That's a very different kind of world. A drastic departure from our common sense view of the world.
But even though most of us find that hopelessly implausible, it is coherent. It seems to be hypothetically possible.
By contrast, if God exists, then everything else depends on God. There are, moreover, Christian philosophers who formulate arguments for aspects of that claim.
In that event, God's existence is fundamentally more certain than the existence of my hands. A world could still exist even if my hands are illusory. That might or might not require some adjustments in what the world is like. But no world of any kind can exist apart from God.