This occurred to me while receiving communion yesterday. I was thinking about why for Melanchthon it wasn't that big a deal to let go of exact language regarding what was going in the Lord's Supper, whereas for Luther denial of the real presence the worst theological crime possible. In the same, I was thinking- why isn't there a doctrine of unio mystica in Melanchthon?
For all my criticism of the Finns, I do think that they're on to something. This is true even if they ultimately execute take a particular idea too far or draw too many conclusion out of it.
So, Luther and Melanchthon ultimately do agree about justification as forensic, as I argued last time. Furthermore, this is very clear from the historical record. Beyond this, they both agree that justification is prior to faith even and creates faith. Consequently, claiming that unio mystica is in fact the same as justification or simultaneous with it is out.
I will agree though that there is a difference between them in how they conceive how the divine-human relationship through faith is conceived. This is what I think the Finns are picking up on and then distorting somewhat. Nevertheless, it leads them to a valid insight that for Luther is fundamentally participatory, whereas for Melanchthon it is not.
Allow me to explain.
Melanchthon studied Aristotle as a young man. This was not the Aristotle of the Scholastics, but he was studying Aristotle in the Greek. He believed that Aristotle could serve as the basis of the program of study at Wittenberg. Most importantly though he bought into Aristotle concept of language and knowledge.
According to Aristotle, we know things by having them impressed on our intellects. This makes an imprint or copy in your mind. To know a thing isn't participatory per se. It means having the effect impressed on you.
Let's think about justification then. If God tells me through the preacher "Your sins are forgiven for the sake of Jesus" and I believe it, is actual presence of Jesus in that faith required? Not exactly. Perhaps I need the Holy Spirit to move me to accept it, but that's it.
What has to happen, is that that piece of information about Jesus did simply has to be impressed on my mind so that I know it.
In the same way, what's the point of Jesus being present in the Lord's Supper if you hold this theory of knowledge? In other words, if assurance of justification as something impressed on your intellect is what your looking for, why not just make the Lord's Supper a means whereby the person of Christ is present and impresses upon your intellect the information that "Jesus died for your sins." Eating flesh and blood then don't matter a hill of beans! And in later life, (for example in Loci Communes 1555), that's about all Melanchthon was willing to affirm about the Supper.
Now, let's move to Luther. Luther is a man of the Bible, but he has also studied Aristotle (though in Latin translation). He's also been greatly formed by monastic piety. These are both important factors.
First, in the Bible, knowledge is always participatory. For example, the word "know" means have sexual intercourse (as we all know). Nonetheless, it also means the act of epistemic comprehension (as it would in English). This is why God uses sacraments throughout the Bible. Eating means knowing, because it means participating in the object of knowledge (hence the tree of the knowledge of Good and Evil- having participating in disobeying God one will now really know evil!). That is to say, knowing a thing is really getting it in you. That's the only way you know it.
Monasticism had worked similarly in Luther's day. The idea that one would enter into the suffering of Jesus by taking up the cross. We can see this in Luther's early writings. He believed that Christian should enter into the suffering of Jesus and have their ego destroyed. Suffering what Jesus suffered, you would really know Jesus and become like him.
We can see this Biblical and Monastic piety reflected in his later theology. This is the entire basis of his method of theology. Meditation, prayer, suffering, is all about experiencing God in this way. Unless God breaks you down and you "suffer divine things" then you can't know God. If you can't know God, then you really can't be a theologian.
Later, after the Reformation breakthrough Luther retains this idea to the extent that knowledge of justification means being "baked into one cake with Christ." Receiving the real presence of Christ in faith, he becomes the thing which impresses itself on the intellect of the believer. He uses the Aristotelian description of form/matter to suggest that Christ is present in the act of faith and imposes himself directly by his presence on the person's mind: "Christ is the form of faith." Faith is a "ring, Christ is the jewel." Receiving Christ with faith, means knowing Christ by mystical union. Christ is of course still for us before he is in us, and therefore FC is correct that for Luther justification is prior to mystical union.
It's important to see this in the LC also. This is why he states that Baptism is a literal death and resurrection. One doesn't really have the promise of baptism based on Christ's death unless one literally participates in it.
Finally, we can see in this perspective why the real presence in the blessed sacrament of the altar was so important to Luther. It is as Sasse said, the "Gospel is the sacrament and the Sacrament is the Gospel"! One does not have Christ in the sacrament present and giving himself to you to be known, then you do not know him and his present self-giving reality.