What is Progressivism's primary source of authority?

While I cannot speak for every progressive Christian, I know for me it is the words and example of Jesus. One of the things I like to tell people is that the words and actions of Jesus are paramount—the rest of scripture is sacred commentary. Now that might sound snarky or arrogant to some, but I think we have to truly ask ourselves who we follow. Most Christians, if we're honest, follow Paul over Jesus. In Christianity—especially Protestantism—we have tended to make Paul of Tarsus our authority when it comes to theology and praxis.

As a progressive Christian, I value the writings of Paul very highly (those that we can view as actually be written by Paul; not the latter reactionary writers who used Paul's name to push through their own chauvinistic and regressive agendas); but as a follower of Jesus, I have to look at the writings of Paul through the lens of Jesus' teachings. Paul, as well as the other writers of the New Testament, were followers of Jesus, just like you and me, and must be viewed as such. If their writings line up with Jesus' teachings and example of God's grace, inclusion, and justice, then follow it; if what they wrote does not measure up to those standards, shake the dust off of your sandals, and move on.

How does scripture fit into a Progressive theology?

While Progressive Christianity may not view scripture as either the transcribed words from God's mouth or a science and history textbook, as are the common perceptions, it is very important and essential to our faith. The Bible is that narrative of our people and our evolving understanding of God and what God desires of us. The Bible has flaws, contradictions, and has a great deal of passages that discuss supposedly "godly" people advocating and doing some very awful and reprehensible things—and I think we should be honest with that and speak against those passages in the Bible that contradict the nature of God as Jesus shared it with us, that contradict the good nature of our own consciences.

How do we know what God's justice really is? What, or who, is defining it?

We see throughout the biblical narrative—in both the Jewish scriptures and in the Christian Testament—that God (whoever or whatever God is) continually calls us to a higher ethic. There is an evolution—not of God, necessarily—but of our understanding of God and God's desire for the world.

God's narrative begins as a tribal God of Abraham and a nomadic people. This early understanding paints a picture of a God that has no problem killing first-born children of another nation to get his/her/its way. This understanding of God reveals a God who commands his followers to invade a land and kill every man, woman, and child. Whenever the Israelites won a battle in Canaan, they attributed it to the will of God and that God was on their side (sounds familiar today, doesn't it?). But God's story doesn't end there. The judges and prophets continue to come and point to a continually progressive (you like how I added that word in here?) idea of God and God's nature until we finally get the prophet Micah to proclaim "What is that God asks of you? Do what is right, love mercy, and walk without pride by God's side" (Mic. 6:8). Then Jesus continues this narrative by pushing the limits of God's grace, justice, love, and inclusion even further. The story doesn't even end there; it is continuing today with you and me.