, I don't.
I've a horrible feeling I may end up reaching the same conclusion, though I'd far prefer it would be otherwise...
Why "horrible"? How can you know what it is like being an atheist?
Actually I think I understand your feelings. I have been very anxious myself, some years ago, about the idea of "losing faith". But to me admitting I didn't believe in "God" anymore was not "losing faith" at all. It was rather "walking by faith", as it were, on an unexpected path. What I loved in "faith", or in the Bible, is still here -- and I feel the idea of God was more of a hindrance than a help to it -- but that's only my view.
Are you suggesting there was no concept of God before the 6th century BC?
Yes, definitely. I mean the concept of "God" with capital "G", which is very different from, though tributary to, the older mythological and polytheistic concept of gods (and goddesses).
Surely even the written record of the Bible is around a thousand years older, never mind the many 'pagan' concepts of God that went before those (and from which many of the characteristics of the Biblical God is allegedly drawn)?
Check any recent critical Bible introduction (from a non-fundamentalist Bible edition for example): you will realize the process of writing the texts which now make up the Old Testament hardly began before the 8th century BC, and that all the former texts were subject to monotheistic rewriting after the Babylonian exile (6th century BC). Still the old polytheistic view, in which Yhwh is just one god among many others, show in many places...
I'm afraid my knowledge of Nietzsche is sadly lacking (my sole 'further education' was attending the Pioneer Service School!). What was his point?
I attended the Pioneer School too instead of going to college, but there's still plenty of time to read when you're out! I suggest his masterpiece Thus spake Zarathoustra (very Gospel-like in style), and also the provocative The Antichrist as an introduction to Nietzsche's thinking.
What are 'the best of philosophical arguments for the existence of God' and where can I find out more about them?
It depends what kind of arguments you are sensitive to. The classical, metaphysical "proofs" of God's existence have been developed by a number middle-age Church Doctors such as St. Anselm or St. Thomas. After the collapse of metaphysics, many attempts have been made at giving a new base for "God". Epistemological in Descartes' Discours de la méthode, moral in Kant's Practical Reason... For one time I was quite fond of Kierkegaard's paradoxical view of God, or Emmanuel Lévinas' humanistic and poetical vision of "God" as the "other's face"... However, I came to realize the definitions of "God" I was mostly attracted to didn't suit the common meaning of the word "God" -- so I finally preferred to confess my atheism, while feeling free to refer to "God" (or "gods") as a metaphor...
Wherever it may lead you, the path of "faith" as I understand it implies reading (preferably NOT WT or fundy stuff), thinking and above all living...