arcanum arcanorum

But this is a totally different idea from that of the seekers of the grand arcanum.
A System of Logic: Ratiocinative and Inductive|John Stuart Mill
Arcanum relative to conception, which takes place though the souls of two married partners be disjoined, 245.
The Delights of Wisdom Pertaining to Conjugial Love|Emanuel Swedenborg
Arcanum respecting the actual habitation of every man in some society, either of heaven or hell, 530.
The Delights of Wisdom Pertaining to Conjugial Love|Emanuel Swedenborg
Totam igitur veritatem et omne divinæ religionis arcanum philosophi attigerunt.
The Sympathy of Religions|Thomas Wentworth Higginson

For instance, the Royal Arcanum met to discuss what could be done about the increased rates.
The Library and Society|Various