How do liberal Christians know the “true message” of the Bible?

Peter van Inwagen, in his contribution to the anthology Divine Evil, accepts that the Bible is morally flawed, but thinks we can know which parts of the Bible to follow and which to ignore. How? Supernatural guidance:

‘But then how can we turn to the Bible for moral guidance?’ Well, if you turn to the Bible for moral guidance, you mustn’t treat it as an essay intended to present a system of morals, as a book like Spinoza’s Ethic or The Critique of Practical Reason. Spinoza and Kant could influence the thoughts and beliefs of their readers only by saying things, only by putting forward propositions and arguments for those readers to consider. God (who is in one sense the author of the Bible) is under no such limitation. He can reach into you and touch your heart and guide your thoughts. And—the Church’s promise is

—he will do this when you read the Bible. He will be present within you and will guide you through its pages, highlighting this passage, awakening your critical capacities when you read that one, creating in your mind a sense that ‘this passage is not addressed to my condition’ when you read a third. He will, moreover, guide you to passages he particularly wants you to read: tolle lege. And, if you encounter difficulties in the text, he will lead you to people—the Doctors of the Church or your Aunt Alice—who will help you to resolve them.

All this is true provided you are willing to be transformed by submitting yourself to the will of God. If you come to the Bible with preconceived moral notions (say, that slavery is morally permissible) looking for ‘proof texts’, you will not only get no moral truth out of it, but you will almost certainly do yourself positive moral harm. But if you have submitted yourself to God’s will and if you read—say—that God has commanded that the children be punished for the sins of the fathers, your reaction will be along these lines: ‘Yes, that’s what seemed self-evidently true to the Hebrews once, that it was right to punish the children for the sins of the fathers, and that that was therefore what God would have told their ancestors to do; with God’s help, we now know better. And their descendants came to know better. This was the mindset that God was leading the Hebrews out of. This is what—with God’s help—the author of Ezekiel knew that the authors of the Pentateuch did not know.’ (pp. 82-83)

This is of course terribly implausible. Were all the countless Christians throughout the centuries, including ones still regarded as great theologians like Augustine and Aquinas, who read the Bible and came away with morally reprehensible conclusions, really not just submitting themselves to God enough?

Van Inwagen’s solution has one virtue: at least it’s an explanation of how we’re supposed to know which parts of the Bible to ignore. It seems to me that most liberal Christians don’t even have that. They insist they know that the true meaning of the Bible is love, but I’ve read the Bible, and variations on “do what God says or else” get way more play than messages of love ever do. How do they