Actually that's another one of the misconceptions about solipsists, that the only thing that they can be certain of is their own personal existence.
For example, I know that you exist, at least as an interlocutor on my computer screen, both of which may only exist within my own mind. But nonetheless, you both exist, and it would be illogical for me to argue that you don't. Following that line of reasoning there are a great many things that I know exist, including the laws by which they behave, because it's their adherence to these laws that allows them to form coherent and recognizable entities. Those laws may be as simple as the Principle of Sufficient Reason, or as complex as Einstein's Theory of Relativity. But they have to be there.
So you see, a solipsist's worldview isn't as simple as it might at first appear, and within that worldview comes the inevitable question of... where did it all come from?
That's a tough one, and I'm apt to be a bit skeptical whenever someone claims to know something that they can't actually know. Now faith can be a wonderful thing, but believing that something is true, and knowing that it's true are two different things.
Therefore I choose to believe in the commandment to 'Love thy neighbor as thyself', because that's a hill that I'm willing to die on... or a cross, if you'll forgive the presumption.