You write you could never be a Theosophist. Well I suppose I could say the same thing, if you're referring to what most theosophists are. But that does not alter the fact that I believe that the principles of theosophy are true, and that it leads to clarity in one's spiritual development. Which means that we [Mondrian & the Catholic painter and his former teacher Schelfhout, after their reconciliation] quite agree on this point. Self-awareness is, in my view, of crucial importance to all human beings. I can understand how the Catholic doctrine may lead to vagueness, but Theosophy, which is a spiritual science, can never do so.
In a letter to Lodwijk Schelfhout, Paris 29 January 1914; as quoted in 'Beeldende Kunst: Opmerkingen over de tentoonstelling van den Modernen Kunstkring.. Der Ploeg (1912)', W. Steenhoff, p. 147