The historical unreliability of the Old Testament
No-one (that I have met, at least) seriously believes in seven-day creationism. We are quite willing to take this as an allegorical account of creation. But this is far from the extent of the departure of the Old Testament from the historical record. Most obviously, there is the whole story of the Exodus, which has no mention (that we know of) in Egyptian writings, and suffers from a distinct lack of archaeological evidence. A tribe of many thousands of people spending forty years in a desert should be expected to leave remains, yet there are none to be found.
A perhaps more worrying problem is that Judaism as a religion did not develop until well after the Jews had settled in Canaan. The OT presents an account in which God gives his laws to the Jews on Mount Sinai, and they take this law with them to Canaan as their dominant belief, but the fact is that at the time they moved into Canaan there were many different beliefs floating around - Judaism was merely a crystallisation of certain of these beliefs.
I'm willing to accept that not all of the Old Testament is meant to be taken literally, but one would expect there to be a clearer division between the sections intended to be literally true (e.g. 1 and 2 Samuel, 1 and 2 Kings, 1 and 2 Chronicles) and the sections intended to be allegorical (e.g. most or all of Genesis and Exodus).
(Over the summer I intend to read The Bible Unearthed by Israel Finkelstein and Neil Asher Silberman, in order to find out more about the historical record here).
The problem of evil
The classic anti-theist argument. I would express it roughly as follows:
(1) There is a God who is omnipotent and loving.
(2) Evil exists.
(3) God could get rid of evil (from God's omnipotence, in (1))
(4) God wants to get rid of evil (from God being loving, in (1))
(5) So why does evil exist?
This challenge to Christianity might perhaps be evaded by certain views of the nature of evil, so instead of evil let's use "suffering". Suffering is something I believe we can all agree is bad but exists. I'm not convinced by the Free Will defence, largely because I'm not convinced that the concept of free will makes a great deal of sense - or rather, I'm willing to endorse a certain compatibalist view of free will, but this does not give us moral responsibility, which is what I believe we actually care about.
(Over the summer I intend to read Be Still, My Soul, a collection of essays about suffering written by various Christians and edited by Nancy Guthrie).
For since the creation of the world God’s invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that people are without excuse.Surely Christianity should be more obvious if this is true? I would love to be able to believe in the truth of the Bible, but so far as I can tell the evidence weighs heavily against it.
This passage becomes even worse in context. It accuses unbelievers of refusing to worship God even though "they knew God". Is it really impossible that someone should honestly not believe in God?
One of the major arguments for Christianity is that it gives us a grounding for moral realism. The argument would run something like the following:
(1) If Christianity is false, then we cannot have objective moral truths.
(2) But we have objective moral truths.
(3) Hence Christianity is true.
I think I'm willing to accept (1): I'm not convinced by any of the secular arguments for moral realism. The problems are (a) that I'm unconvinced of (2) - I'd be quite willing to accept morality as no more than a useful fiction, and (b) I don't really see how Christianity succeeds in providing a ground for objective moral truths. What is it about God that gives His commands the force of morality? Robert Merrihew Adams has suggested that it is His loving nature, but this seems far from sufficient. I love my brother, but this does not give me any kind of authority over him.