Eckhart was not only a scholar, a preacher, a spiritual leader, but also a man who believed in the importance of the individual experience.
His doctrine was about academic freedom which invited individuals to not being constrained in their knowledge by educational institutions. Knowledge for him goes beyond written words and books. He tells his readers in the prologue of one of his major works, the "Opus Tripartium," not to rest on the apparent sense of his words, but to extent great effort to apprehend the true meaning.
Religious Freedom, in the sense that spiritual experience should be individual, free from human agencies where the only relationship to the outside self is God.
And Pluralistic Society, that invited diversity because "we are one with God, and nothing without him."
McLuhan in The Medium is the Massage quotes Eckhart, and I would argue that this passage does not demonstrates Eckhart's doctrine. The quote goes,
"Only the hand that erases, can write the true thing"
Before I continue I have to say that this quote is taken out of context (form sources that I could not find and therefore wonder if is even Eckhart's). It is important to know the context because in the Medieval times Eckhart was accused and condemned for preaching against the Christian Doctrine and what happened was that he was not understood, they were only taking sentences instead of looking at the whole paragraph.
Going back to the quote. If Eckhart said this (giving McLuhan the benefit of the doubt for not citing his quotes), I think he would most likely mean that the hand, representing the human individually, that "erases" or moves away from preconceived constructions can "write" construct, build, or create the true thing. In other hands, to be able to know the truth, you need to move away form what exists and go beyond that. And the only reason he would promote writing (the true thing) anyways is to teach. Although he differentiated educated from intelligent (meaning that everybody is intelligent and is able to reach pure knowledge), he thought important to teach this "pure" or true knowledge to everyone.
The reason why I think McLuhan is not using this quote appreciating Eckhart's doctrine/ philosophy is because Meister Eckhart would say that the message itself is the most important than the written words, than the text, than the book, than the medium.
The other thing to do is to use Eckhart's work to theorize this quote above in McLuhan's terms.
In that case I would say that In order to "write", construct, and create the true thing (the thing that is validated as important, or becomes the framework of understanding) you need to be able to "erase," delete, or deconstruct. And that is where the media comes handy. You need a powerful tool that can delete constructed/preexisting conceptions in order to be able to create new ones. In that way, the medium massages your brain to get the message by erasing the old and writing the new, or making enough space to add in what it wants to add in.
Even if what I just wrote makes sense, I still wonder if McLuhan understood Eckhart's message, or if he just applied it in a secular way. Part of me wonders if he just knew this popular saying and had to add it on his book because it made sense to what he is saying.
The world will never know. In the mean time I had fun reading tons of sermons by Eckhart trying to find that stupid quote and understand his philosophy.