Wars were caused by religion: Wars can be caused by anything, especially things people have strong and polarising beliefs about. Just because people have wars about religion doesn’t mean religion is bad. If anything, it is reflective of human nature, which is a whole other thing. In short, its a people problem, not a religion problem.
Evil is done in the name of religion: Just because evil is done in the name of religion doesn’t mean religion supports that evil. Someone might kill someone in the name of Australia, but that doesn’t mean Australia supports that act. Some Australians might, but you can’t generalise an entire country, nor a religion with such a diverse and divided following.
If you were born somewhere else, you wouldn’t be Christian: While this is true, this doesn’t in any way disprove Christianity. Christianity, compared to other religions, has an incredible spread. While other beliefs may be geographically restricted, Christianity is not. It has presence worldwide, and while there are, of course, some gaps, this is because of human error, not God's. While God directs and instructs, we are ultimately an imperfect instrument. Due to our imperfections, God will have to judge based on our individual circumstances. If someone didn’t have the chance to be a Christian or follow him, he will judge them as such. He is fair and just.
There’s no scientific proof that God exists: Not necessarily, while there is no scientific proof of God’s existence, there are many pointers or hints to His existence throughout creation. This is the foundation for the teleological argument and the cosmological argument, among others.
There’s no evidence of anything supernatural: The Gospels, Paul's letters, Josephus' references to Jesus’ miracles, Tacitus, and early church members. You cannot deny that, at the very least, Jesus existed, and he was extraordinary. While this, in and of itself, is not proof of the supernatural, it should be clarified that it is a suggestion at the least.
Emphasise that we have multiple independent sources (Paul, the Gospel writers, early church fathers, non-Christian references like Tacitus and Josephus). Even sceptics admit something extraordinary happened around Jesus.
The Bible has contradictions: Of course, it has contradictions; it would be concerning if it didn’t. It is millennia old and has detailed accounts of various complex historical events. In many cases, writers were writing decades after the actual events.
The Bible supports slavery, incest, etc. No, it doesn’t. Just because something is in the Bible or people who wrote the Bible believe in it, doesn’t mean God or Christianity supports it. The distinction between Judaism and Christianity is also important to make. While Paul and even some of the disciples do support slavery, this doesn’t mean God supports it. It means humans aren’t perfect, and humans wrote the bible; even if God guided them, there will still be human influences throughout.