James calls mystical experiences ineffable, which means that they cannot be expressed in ordinary language. The author of the mystical ancient Chinese text Tao Te Ching expressed this idea when he wrote, “Those who know do not speak. Those who speak do not know.” The author violates the rule in stating it.

The Tao Te Ching and other mystical tracts seethe with these sorts of Godelian, “this-sentence-is-false” paradoxes, and so does Tractatus. Wittgenstein writes, “Not how the world is the mystical, but that it is.” He elaborates: “We feel that even if all possible scientific questions be answered, the problems of life have still not been touched at all. Of course there is then no question left, and just this is the answer. The solution of the problem of life is seen in the vanishing of the problem.” Even when the world has been thoroughly explained by science, Wittgenstein seems to be saying, it hasn’t really been explained at all. The answer to the riddle of life is that there is no answer.

and

[Wittgenstein] seriously contemplates becoming a monk. It all started from William James’s Varieties of Religious Experience, and grew (not unnaturally) during the winter he spent alone in Norway before the war, when he was nearly mad. Then during the war a curious thing happened. He went on duty to the town of Tarnov in Galicia, and happened to come upon a bookshop, which, however, seemed to contain nothing but picture postcards. However, he went inside and found that it contained just one book: Tolstoy on the Gospels… He has penetrated deep into mystical ways of thought and feeling, but I think (though he wouldn’t agree) that what he likes best in mysticism is its power to make him stop thinking.

I like that final sardonic twist. I found this quote from Russell in a 2009 essay on Wittgenstein’s mysticism by philosopher Russell Nieli. Nieli warns that most philosophical commentary on Tractatus “has focused on the book’s technical system of logic and language with little concern for its overarching moral and spiritual thematic.” Tractatus “remains misunderstood largely because of interpreters’ failure to appreciate the importance of the mystical and the ecstatic as they are interwoven into the text.”

and Wittgenstein himself said

My propositions are elucidatory in this way: he who understands me finally recognizes them as senseless, when he has climbed out through them, on them, over them. (He must so to speak throw away the ladder, after he has climbed up on it.) He must surmount these propositions; then he sees the world rightly. Whereof one cannot speak, thereof one must remain silent.

Quotes drawn from this wonderful essay by John Horgan on May 25, 2018:

https://blogs.scientificamerican.com/cross-check/was-wittgenstein-a-mystic/