I know this, that if genesis is a collection of mythical tales we have no Christianity.
That's simply not true at all. Nothing is lost that Christianity
requires. Maybe we should take stock of exactly what
would be lost?
Young Earth Creationism is lost. Genesis would still teach creation, just without the silly insistence that the world is 7000-ish years old. Can't say I'd be sorry to see
that go.
Augustine's formulation of the doctrine of Original Sin is lost - the idea that everyone is guilty of Adam's sin. The Bible would still teach that everyone sins, and that men by their nature tend to work iniquity... just without the gross insistence that God condemns innocent babies who die to an eternity in hell because of something that someone else did. This seems like a net positive to me.
The idea of a global flood that killed all but 8 people in precisely 2386 BC is lost. The Bible would still teach that God periodically judges the nations throughout history, destroying the wicked and saving the righteous. Noah's covenant with God would remain intact, too.
The idea of Abraham and his literal, physical descendants as determined by genealogies being God's chosen people throughout history probably needs to be re-understood. Abraham's covenant with God remains, and he is still a father of nations, but his descendants need to be reckoned spiritually, according to adoption. Actually, this needs to happen regardless, because it's what Jesus and Paul and the New Testament teaches. So all we lose here is a common mis-understanding of the Bible.
It turns out that understanding Genesis doesn't dismantle Christianity at all.