How do you know leprechauns don't exist?
As far as Christianity, answer this:
Christianity is the belief in a perfectly just God who demands the death of an innocent life to pay for crimes of others before he'd forgive them.
How on Earth does punishing somebody for something he didn't do meet any imaginable definition of justice?
You will probably say, "God requires perfect justice, and sin deserves punishment, so somebody had to take the punishment." No, sin doesn't deserve punishment; sinners would deserve it. We don't punish crimes, we punish criminals for committing crimes. We don't just pull somebody off the street and punish them for a crime that occurred if we can't solve the crime just because a punishment has to happen for it, whether it's the actual criminal or somebody else - and if there were a country that did that, you would surely call that unjust, right? It's only justice if the actual offending criminal is the one being punished.
You might say, "But Jesus volunteered for it!"
Okay, then lets' take this scenario: Somebody rapes and kills your closest female family member, and gets away never to be caught. You will say, "But I want justice for this crime!" So, your next closest family member (daresay your son?) says, "Hey, I will volunteer to take this murderer's crime upon myself, and I will go to prison in his place. This way, the penalty has been paid, justice served." Would you accept that and call it justice? Of course not. No sane person would. It makes no sense, in any context, to punish an innocent person for the crimes of another. Yet this is exactly the scenario with the Jesus sacrifice. The only thing Christians try to pull in response to this is usually, "We can't understand God's ways, his standards of justice may be different from ours" and when they do that, they're admitting that they don't care whether or not their beliefs make sense, and also that they think God's justice is lesser than human justice, because they aren't in favor of a "punishing the innocent" policy on Earth. If it's perfect justice, why aren't they in favor of it? Do you care whether or not your beliefs make sense? And if so, how can you make sense of this narrative of punishing the innocent?
It only becomes stranger when the innocent person is another version of the very being demanding it, sacrificing himself to himself, and even sillier when the punishment of "death" is more like a three-day coma...yeah...can you make sense of any of this?