Christianity Problems:
At the heart of Christianity (regardless whether Presybetarian, Catholic, or Jehovah's Witness) is that Jesus is our Savior. Some folks believe that Jesus was just a great teacher, but those folks aren't Christians. To be our Savior, Jesus must be perfect: God can't be wrong. Hence, if Jesus was wrong about just one of his teachings, he can't be the Son of God. That's a pretty high standard, but you should have high standards if you're claiming to be divinity.
So what is Jesus's flaw in his teaching? Two examples before my conclusion.
1. Remember that bit of "Turn the other cheek?" Basically, Jesus says if somebody hurts you, you don't automatically hurt him back (no "eye for an eye, tooth for a tooth.") Instead, you just allow him to keep hitting you. If he steals from you, let him keep stealing from you.
The problem with this teaching is that it doesn't work. It might work sometimes (say in Mahatma Gandhi's case) but overall, it's a pretty stupid teaching. I used to "turn the other cheek", not out of love of my enemies, but mostly because it really didn't hurt that much when people slapped me. And besides, it would take effort to try to hit back. So I just turned the other cheek. The result? It just pissed off the aggressor even more. Effectively, I was just saying "You're so unimportant, it doesn't even bother me when you try to hurt me." (This was true.) But this pissed off the aggressor, and so to achieve importance, he just kept on trying to hit me harder.
Essentially, turning the other cheek just traps one in a cycle of abuse. They hit me. I do nothing. They hit harder. I still do nothing. They hit even harder etc. This is bad for me (obviously) but it also is bad for the aggressor. Every time they hit harder, they feel worse for hitting. Do they eventually realize that hitting me just makes them feel worse? Sometimes. But more often, they just keep hitting and feeling worse and hitting. Hence, "turning the other cheek" is a great strategy if you want to make the other person feel like grap. But that's not my goal. My goal is to make the world a better place, and turning the other cheek just increases the amount of grappiness in the world.
What's the alternative? If someone tries to hit you, you block them, slam them hard into the ground, and apply an arm lock. Now that he's in your power (and he respects you) you figure out why he felt the need to hit you. And try to solve that problem so he doesn't need to hit you anymore. Maybe he's trying to hit you because you scratched his car. Offer to pay for the repair. Or maybe he's trying to hit you because his mother didn't love him enough, and he's taking his rage out on the rest of the world. Well then, take him to counseling and try to help him find self esteem. My point is don't just take the hitting and hope the aggressor figures out that he's wrong, you should stop the hitting and then help the aggressor figure out what's bothering him and help solve his problems.
2. There's a bunch of hypocritacal Pharisees who are giving to the church. Announcers say "Pharisee Joe is giving $1000." "Pharisee Bob is giving $2000" "Pharisee Harry is giving $10, 000" And then this old lady drops in $.02. Jesus says "This old lady is awesome because she doesn't give out of excess, she gives out of need. Her contribution is worth the most."
While a charming story, this teaching is stupid. By giving her last $.02, the old lady just increased the burden on society to take care of her. She also might starve. She has just managed to increase the amount of pain in the world.
Yeah, the Pharisees are shallow and self centered, but guess what? That $1000 will still feed hundreds of hungry people. It would be better if they gave quietly, but it's better to give loudly than not give anything at all. And it's better to give out of excess and decrease the amount of pain in the world, than to give out of need and increase the amount of pain in the world.
So what do examples 1 and 2 have in common? They both praise intent over utility. Jesus says one should intend to help your enemy and the poor. Even if your actions don't have those results, it doesn't matter. Intent is all important to Jesus. No where in the gospels does he ask folks to use their intellect to determine if their actions actually bring about the desired results.
And that's the problem with Christianity. This single flaw in Jesus's teachings has compounded into more pain over the last 2000 years. The Crusades. The Irish Christian terrorists. "I'm intending to make the world a better place- never mind that I'm bombing 8000 innocent people to get there."