What can we do as a nation to actually FOLLOW what is in the bible more?

This is exactly why the bible is arguably the single most harmful piece of literature to ever be mass-published (it and the koran are basically side by side in that respect). So many christians and religion apologists claim that passages like these are no longer relevant, or that the old testament was canceled out by Jesus. If that is so, why do christian bibles still contain the old testament? If it has been rendered irrelevant to your belief, why is it still quoted by priests and pastors of so many christian sects?