What is the basis for the idea that the Song of Solomon is about the relationship between God and the church?

I am familiar with the interpretation of Song of Solomon which suggests that it is love poetry written by (or at the behest of, or in honour of, or later attributed to) King Solomon and that it is about human romantic love. I recently found out that as early as Origen it has also been interpreted as being about the relationship of God and his people. I am struggling to find out what case was made for this interpretation of Song of Songs. What is the case that song of Solomon is about the relationship between God and the church and who came up with this interpretation?