Rabbi Yosef Kapach was the grandson and disciple of the founder of the near-dead Dor Deah movement, Rabbi Yichya Kapach.
I don’t know if this tale is reliable, but the story is told Rabbi Shlomo Zalman Auerbach zatzal was honored as Mesader Kiddushin, and when he noticed Rabbi Yosef Kapach zatzal was to be one of the witnesses, he honored Rabbi Kapach, instead, because Rabbi Kapach may have denied the validity of the Zohar/Arizal tradition (I am unaware, myself, of perfect evidence for this, in spite of his general fearlessness to speak [and write] his mind), and/or maybe this renders him a heretic.
If this story is true, it only demonstrates further the absurdity of the position there are “principles” of faith in Judaism. The rabbi is knowledgeable enough to arrange the marriage, but cannot testify to it? He has enough Yir’as Shamayim to remember his learning and certainly seems to keep all observable mitzvos, but because he disagrees with someone or something revealed far later than the age of the sealing of the Talmud, he is really an imposter?!
In my view, if you keep mitzvos, you are a kosher Jew. That’s about it. The Achronim have already disagreed with the Rambam’s “13 Principles”. Only in our times has this been “forgotten”.
Read more about this here on page 10, and on for the interesting claim the Rambam’s position, too, has been grievously misread.