The resurrection of Christ
This is crucial to the whole religion - no resurrection, no Christianity. The only evidence we have for this is the Bible, and that's a bit self-contradictory.
What were the last words of Jesus? Three gospels give three different versions.
Who buried Jesus? Matthew says that it was Joseph of Arimathea. No, apparently it was the Jews and their rulers, all strangers to Jesus (Acts).
How many women came to the tomb Easter morning? Was it one, as told in John? Two (Matthew)? Three (Mark)? Or more (Luke)?
Who did the women see at the tomb? One person (Matthew and Mark) or two (Luke and John)?
Was the tomb already open when they got there? Matthew says no; the other three say yes.
So we have several different versions. If you put this up in a court of law, the differences between the witnesses would be enough to have it thrown out of court.
So how do you explain this? There's a few ways. The best is to explain that the differences between the witnesses, proves that there was no collusion, and therefore strengthens the story.
The Noah flood
There are so many ways that this can be ridiculed, I shan't bother listing them here. The best answer is to explain that the story is allegorical. Not everything in the bible is meant to be taken literally; consider, for example the story of the Prodigal Son. No-one is suggesting that the story actually happened, it's an explanatory device. Likewise the Flood.
If, however, you believe that the Noah flood was actually and literally true, I don't think I can help you; you'll quickly find yourself up the creek without a paddle.
But the problem with allowing that parts of the bible are allegorical, is that you run into the question of "Which parts, and who decides?" That's why many Christians prefer to stick with "It's all totally true"
The crimes of God
Atheists might bring up the three great crimes of God. The Noah Flood, where (apart from those in the Ark) God drowned every man, woman, child, baby, cow, sheep, goat, cat, dog etc etc. The second being Sodom and Gemorrah and the third being the Slaying of the Firstborn in Egypt.
Again, a good defence is that all three stories are allegorical. That's no good if you're a bible literalist, so try this.
The Noah flood was justified because all the men and women were evil. The children and babies would have become evil when they grew up, so it's justified in killing them too. The animals were tainted by the evil of the people (including the wild animals), except the few that made it to the ark, they weren't tainted, so they were rescued from the flood.
Plus, God made them, he is entitled to kill them.
Plus, God could take them all to heaven, although if they were all wicked, that's hard to swallow.
The same responses can apply to the other great die-offs.
Here the question is put as follows. "God loves you so much, he made Hell for you in case you don't love him back". The answer is simple. You don't have to go to hell, just love God.
Another question, "Is it moral to exact an infinite penalty for a finite crime?". The answer is, God is always moral.
God will forgive all sins (except blasphemy, obviously). The objection is that the person sinned against isn't being consulted here. The answer to that is that the sin is only forgiven if the repentance is sincere, and sincere repentance includes restitution of damages.
A common question is "Why do you believe in God" (or Jesus). Your best answer is if you've had a personal revelation (such as a Near Death Experience, or a vivid dream. Failing that, if you "Just know in my heart" that's pretty hard for them to answer. Citing the Bible as evidence will inevitably lead you to having to show why you think that the bible is true, and that's really difficult, especially if you believe that parts of it are metaphorical.
The Old Testament is a necessary part of the Bible, because Christianity needs to have Adam and Original Sin; without the OT, that isn't there. Also, the prophecies of the OT foretell the coming of Christ.
The problem with the OT, is all the laws. Like the ones on mixed fibres, on not eating pork or shellfish. And worst of all, the verses in Exodus that regulate slavery. And the thing about killing witches, and stoning people who work on Saturday, and stoning Homosexuals. So you have to say "That's the OT, and that was all replaced by a new covenant because of Jesus". But that really means that you can't rely on the Ten Commandments, because that's OT. And worse, Matthew 5:18 says "For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished." Which means that the OT laws still apply.
So if the OT comes up, just change the subject.
Another difficult one. Most Christians are against homosexuality. If you're one of them, then be ready to defend your position.
You can't cite Leviticus, because that's OT and if you do, you have the problem of pork and shellfish. You can't cite "unnatural" because so is wearing clothes. I think you just have to go with "Because I don't like it", but then you have the problem of other people not liking ice cream, and they aren't telling you to stop eating it. No, I think your best bet is either to say "I'm cool with gay" or else change the subject.
The end times and the Second Coming
Every so often, perhaps a couple of times per year, someone announces that the End Times are imminent, and gives a date. Big mistake, because when that date comes and goes, they look foolish. If you're unlucky, then at the time of your conversation, such a date will be imminent. I would strongly advise you to disassociate yourself from that date, otherwise you too will look foolish shortly. The reason you might give is "They aren't real Christians", for example
But that leaves the problem of the Second Coming. Matthew 16: 27, 28, “For the Son of Man is going to come in the glory of His Father with His angels, and will then repay every man according to his deeds. Truly I say to you, there are some of those who are standing here who will not taste death until they see the Son of Man coming in His kingdom.". That was 2000 years ago, and he hasn't returned. Mark 13:26-30 and Luke 21:27-32 say the same thing.
This is a difficult one to defend. The early Christians thought that the end times were imminent and said so. I think you have to go with "It's allegorical".
The prophecies of the coming of Christ
In Isaiah, there are many fulfilled prophecies, but there's also unfulfilled prophecies in the OT, such as the lion lying down with the lamb, and beating swords into ploughshares and making war no more. Obviously, that hasn't happened, and the person you're talking to will cite that as evidence that the Messiah hasn't come yet.
The counter to this is to explain that this will all be put right on the Second Coming.
The virginity of Mary
The question might be put "How can a virgin bear a child?" Well, that's an easy one - it's a miracle.