For today's average Christian, it is commonplace to read Song of Solomon as a prophetic picture of Christ, as the Bridegroom, and the Church, as the Bride.
But where did this view have its beginning? Who was the first early writer to interpret the book Song of Solomon as a prophetic picture of the relationship between Christ and the Church, and not only between God the Father and Israel?
I've heard that Origen wrote something on that matter, but I couldn't find any reliable source.
I've checked out Ann Matter's The Voice of My Beloved: The Song of Songs in Western Medieval Christianity and R. A. Norris's The Song of Songs: Interpreted by Early Christian and Medieval Commentators and found that Gregory of Elvira and John Cassian were maybe the first ones to systematize this view to the whole book of Songs, while earlier writters had this insight into one verse or another.
But I had a hard time finding a further reference to their writings.
Related question: What is the basis for the idea that the Song of Solomon is about the relationship between God and the church?