From The Experience of No-Self by Bernadette Roberts ...

Perhaps the only philosophy or theology that can help
us cross the stream is one that admits: when you have
learned it all and lived it thoroughly, then you had
better get ready to have it all collapse when you
discover the highest wisdom is that you know nothing.

It is said that St. Thomas Aquinas, after writing his
masterful tomes on Christian theology, suddenly had an
experience of God that so silenced his mind that ever
after, he never wrote a single word. In other words,
St. Thomas literally fell outside his own frame of
reference when he came upon "that" which no mind can
comprehend nor pen describe. ...

It seems that ultimately we must go beyond all frames
of reference when the Cloud of Unknowing descends, and
all the thrashing around looking for a life preserver
won't do a bit of good.

Nevertheless, I now see a possible line of travel that
may be of use before crossing the stream. It would be
to start with the Christian experience of self's union
with God, whereby we loose the fear of ever becoming
lost -- since we can only get lost in God. ...

But when the self disappears forever into this Great
Silence, we come upon the Buddhist discovery of
no-self, and learn how to live without anything we
could possibly call a self, and without a frame of
reference, as we come upon the essential oneness of
all that is.

Then, finally, we come upon the peak of Hindu
discovery, namely: "that" which remains when there is
no self identical with "that" which Is, the one
Existent that is all that Is. ...