Borg wrote 21 books. I've often recommended his books, especially The Heart of Christianity. His two recent book with John Dominic Crossan, The First Christmas and The First Paul, shed new light on both subjects.
Borg was raised as a Lutheran in North Dakota. Later in life, he became an Episcopalian. (His wife is an Episcopal priest.) Borg always claimed his Lutheran heritage. When he asked how many Lutherans were at a recent conference, he said, "I have half my hand up."
Sometimes, I regret to say, the Lutherans were a negative example. He still remembered the parish pastor from his youth who pronounced the forgiveness of sins all the while shaking his finger at the congregation. This had the effect of making the good news feel not-so-good.
Borg's question was how to interpret the Christian faith for people living the modern world. This is the task of modern theology, which Marcus Borg understood, and why he chose that task for his life's work. Dietrich Bonhoeffer tried to do the same, as did Teilhard de Chardin and several others.
I'm honored to have been acquainted with him and blessed to have heard him speak on a number of occasions. He was a gentle and learned man of unfailing graciousness who made an immense contribution to both church and world. May he rest in peace and rise in glory.