In Eph 5:31, Paul quotes from Gen 2:24 –

‘For this reason a man will leave his father and mother and will be joined to his wife, and the two will become one flesh.’

The apostle adds, in the next verse:

‘This mystery is great—but I am actually speaking with reference to Christ and the church.’

Daniel Blanche comments:

‘I do not think the Bible presents marriage as a metaphor for the gospel.  As I understand it – and I confess freely that I am no literary theorist – metaphor is a way of relating two things or concepts which have no (or at least no necessary) real (that is, ontological or conceptual) connection; it is allusive, linking two things which are not directly linked in order to shed light on one or the other.  Metaphor is limited in what it implies, and is essentially linguistic.  It does not imply or establish any real or ongoing link between the two items.  Rather, it uses language to appropriate one thing or concept as a means of ‘opening up’ another.’

But the relationship between marriage and the gospel is real, not merely metaphorical:

‘In Ephesians 5, the apostle Paul is not casting around looking for a great illustration of the gospel and arriving at marital love.  Nor is he looking about for some justification for the institution of marriage and chancing upon the gospel – that direction of thought would be impossible for him.  Look at the way he uses the quotation from Genesis.  His argument depends on there being a real link between human marriage and the relationship between Christ and his church.’

This throws light on, among others things, our understanding of the Song of Songs:

‘When Scripture tells us that the ‘one flesh’ relationship of husband and wife is oriented towards the relationship between Christ and the church, that opens up the reading of – for example – the Song of Songs as a beautiful representation of spiritual communion with Christ.  The question ‘is this about marriage or is it about Christ’ is, in the end, a non-question if human marriage is itself always related to Christ and his gospel.  We can read the Song in a perfectly natural way as a collection of beautiful human love songs, and indeed as a celebration of erotic love, and therefore as having reference to the relationship between Christ and his people.’