The fact that it is provocative for a Christian to say something like "Some of the Bible is hard to believe" indicates that the majority of Christians think there is an obligation to maintain the opposite--that everything in the Bible is easy to believe. I think that is a serious problem for at least two reasons. As a Christian, if I insist that everything in the Bible is easy to believe, I immediately appear to be either A). completely out of my mind, or B). Existing on some unattainable, super-spiritual level. In both cases, the problem is that it makes Christians into something that "normal" people cannot relate to. The first (appearing out of our minds) has to do mainly with the way non-believers see us. Now, before you start chastising me for "caring what people think," or telling me that "the World is supposed to think we are crazy," please just try to understand what I am driving at. Because I'm not saying something controversial. If a non-Christian were to come up to you and say "Some of that stuff in the Bible just seems crazy, isn't it ever hard for you to believe?" and your response is "Nah man, it's all easy to believe, makes perfect sense to me, you just gotta believe," you have instantly made yourself a complete alien to that person. At that point, the invitation to follow Christ sounds, to them, a lot like, "Abandon all reason, embrace fairy tales, and allow yourself to be brainwashed until you believe stuff that no rational person could ever believe."
Likewise with the second (unattainable super-spiritualism), I would argue that deep down inside, the vast majority of people in the Church really do find some of the things in the Bible difficult to believe. Now, that immediately causes a problem of cognitive dissonance. That is, the disconnect between the reality that deep down they find some things in the bible difficult to believe, and the feeling that they must maintain the unspoken standard of either never saying so, or publicly having to say that it's easy to believe. Additionally, when what they get from their pastors, elders, leaders, priests etc, is essentially, "It's not hard to believe, don't question, just have faith," they feel like the problem is that they must be on some inferior spiritual plane of existence--and if they could just "level up" it would all make sense. That leads a lot of people into a discouraging cycle that often ends with the person walking way from the faith altogether because they never reached that "level," and conclude from there that it must all be baloney. The whole "house of cards" comes crashing down because they couldn't come to grips with the story of Noah's flood, or because a history professor said there is no evidence for the exodus from Egypt. And when someone tells them "you just have to believe," or "have faith," it only confirms to them that Christianity is a delusion.
As I said, the problem in both cases is that it makes Christianity into something that "normal" people cannot relate to. When you are sharing Christianity with them, you might as well be saying "I have dedicated my life to eating rainbows because the great squirrel spoke to me in a dream." Again, please don't fire back at me with "Christianity isn't about people 'relating' or 'understanding,' it's foolishness to the unregenerate World! Repent now, and stop trying to herd the devil's goats!" Come on. Give me a little bit of the benefit of the doubt here. I'm not even talking about on the spiritual level. I'm just talking about on a very, very basic human level. The truth is that Christianity exploded onto the world scene, and continues to thrive, precisely because it IS something everyone can relate to. It is good news of something that really happened, was really witnessed, completely changed human history, and can really be looked into, even today.
At this point you might be asking, "Okay, so what then?" The answer is simple. Since Christianity is not based on an idea, but on an event--the historical claim that a real man, Jesus of Nazareth, was really alive, really dead, and really alive again--it can handle your most brutal intellectual honesty. There is no reason to feel like you have to pretend that you have never questioned a story in the Bible, or even the whole thing. By the way, if you haven't that is great! Even so, many people, I'd even wager the vast majority of people, have wrestled with believing some of the Bible at some point. But that doesn't make it untrue. That seems to be what people are afraid of. If I admit that some part of the Bible is difficult for me to believe, then it will suddenly cease to be true. But does that even make sense? Think about it, there are people who doubt that America really went to the moon. Just because they doubt it doesn't do anything to change whether or not it actually happened. Likewise, if the Gospels are a reliable source of information about the life, death, and resurrection of Jesus of Nazareth (and I am convinced there are very good reasons to think that they are)--that is, if Jesus really is who they say he is--then Christianity is true, and that doesn't change just because you have a hard time believing that Noah built a giant boat when he was hundreds and hundreds of years old. And here is the real kicker--the Gospels tell us that Jesus knew and accepted the Old Testament--which means, difficult as many of the stories and passages may be, Jesus believed them, and didn't ever feel the need to qualify it. Does that make them easier for me to believe? No. But it does give me reason to TRUST (that's what faith means by the way) that they are true, because of who Jesus is.
So here is my suggestion: If someone tells you that some parts of the Bible are really hard to believe, you can agree with them, and use that as a jumping off point to talk about Jesus and the evidence for his resurrection. If you think that is just terrible of me to say, I'd ask you to consider that God the Father himself, when giving the Law, chastising the people of Israel for lack of trust, or asking them for obedience, would often remind them, "I am the Lord your God who brought you out of Egypt" (Ex 20:2, Deut 5:6, Num 15:41 etc). Or consider that Jesus said, "Believe me when I say that I am in the Father and the Father is in me; or at least believe on the evidence of the works themselves" (Jn 14:11). My anchor is Jesus and his resurrection. It all goes back to that. If that is true, then Christianity is true. If it didn't, it's false. And that seems to be the way the Apostle Paul felt about it when he said, "if Christ has not been raised, then our preaching is in vain and your faith is in vain" (1 Cor 15:14). I trust in him, and what he has done, not my own ability to be able to believe everything instantly or easily.