I think the very best key by which a Christian can interpret Scripture is to interpret Scripture the way that Jesus did! It almost sounds too simple, doesn't it? Yet, ironically, this has not been the norm at all. So what do I actually mean by such advice?
First of all, Jesus approached the Hebrew Scriptures with the assumption that God had been speaking to humanity through an entire history of dialogue with his people Israel. (The universal message always begins with a very concrete situation or what has been called "the scandal of the particular.") Jesus heard the voice of God inside of one concrete context, exactly as we all must do. But he also had the added advantage of the Jewish practice of midrash. Midrash is a way of interpreting Scripture that fills in the gaps, questioning and imagining a multitude of interpretations possible. Midrash allows the text and the Spirit of God to open up the reader, instead of closing down the possibility of being changed by latching onto one final, closed, and forever certain interpretation.
Unlike Christianity's post-Reformation and post-Enlightenment approach to Scripture, midrash does not look for the one and only interpretation. Jewish commentaries on Scripture invariably say, "It could mean this, and it could mean that, and let's think about it this way for a while?" This open-horizon approach was still in vogue for most of the first 1300 years of Christianity, where as many as six levels of interpretation and numerous levels of truth were perceived in any one Scripture text. We have actually gone backward with the recent and childish notion of "biblical inerrancy," where the one inerrant interpretation always just happens to be mine!
Pope Francis is returning us to the much older and more mature Tradition in his notion of a "hierarchy of truths" (Joy of the Gospel). Not all truths are created equal, or of equal importance. Something might be true merely on a psychological level or a historical level or a mythological level. For some sad and illogical reason, fundamentalists think the historical level is the "truest" one. "Did it really happen just that way?" That is actually one of the least fruitful levels of meaning. Even if it did happen just that way, our capacity to understand even that truth is still filtered through our own cultural and personal biases, which are largely unconscious. There is no such thing as a value-free interpretation of anything. It does not exist on this earth, but only in the perfect mind of God.
Jesus is presented in the Gospels as a classic wisdom teacher. The four Evangelists allow him to draw from the wisdom of other traditions (e.g., Luke 16:19, the story of the "Rich Man and Lazarus" is a retelling of a common Egyptian and Mid-Eastern story), and also to create his own stories, which we waste time trying to prove if they "really" happened just that way. Jesus did not actually quote the Hebrew Scriptures very much, but when he did, he clearly took liberties with them! For example, in John 10:34, Jesus says to the Jews, "You are gods!" This is hardly the meaning of Psalm 82:6, which he is referring to. If we really want to follow his lead, we must begin with Jesus' assumption that a loving God is trying to communicate with us, and even through each little story, verse, or parable. I think that is what Jesus means by blessing "purity of heart" and that these are the ones who will "see God" (Matthew 5:8). If you don't interpret a text with a pre-existing condition of faith and love, your egocentricity, your agenda, and your anger can always be presumed to be in charge, and you will interpret the Bible in whichever way you want.
Jesus selectively emphasizes the texts that reveal his God as good, faithful, inclusive, and merciful, and he creates stories--and concrete healing events--to communicate that very point, such as The Good Samaritan, The Prodigal Son, and The Publican and the Pharisee. Each of these stories and healings are true on many different levels. That is exactly what makes them inspired stories! Jesus consistently ignored passages that reveal his God to be punitive, exclusionary, imperialistic, small, or tribal. Such a biased reading, which is to share in the chosen biases of Jesus, will be endlessly fruitful and life-giving for the individual and for all the world. For the "dirty hearted," the merciful passages will just be something to ignore or argue about, and the punitive passages will be marked with a florescent high-lighter. Pope Francis insists that mercy is at the very top of the Christian hierarchy of great truths, and everything falls apart whenever mercy is displaced by anything else.