What is the origin of the interpretation of the Song of Solomon as a picture for Christ and the Church? 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.
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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?
Origen indeed suggested this connection, in his Commentary on the Song of Songs (~AD 245):


  This book seems to me an epithalamium, that is, a wedding song, written by Solomon in the form of a play, which he recited in the character of a bride who was being married and who burned with a heavenly love for her bridegroom, who is the Word of God. And deeply indeed did she love Him, whether we take her as the soul made in His image, or as the Church. (Prologue)


However, Tertullian is probably earlier (~AD 210), as he quotes Song of Solomon 4:8 in connection with Christ and the Church in Against Marcion:


  This spouse Christ invites home to Himself also by Solomon from the call of the Gentiles, because you read: "Come with me from Lebanon, my spouse." [...] it was from idolatry that He betrothed Himself the church. (Book IV)


But the big winner is Ignatius (d. ~AD 100), as he quotes Song of Solomon 1:3–4 in the Epistle to the Ephesians:


  For this end did the Lord suffer the ointment to be poured upon His head, that His Church might breathe forth immortality. For saith [the Scripture], “Thy name is as ointment poured forth; therefore have the virgins loved Thee; they have drawn Thee; at the odour of Thine ointments we will run after Thee.” (Chapter XVII)