Most people who think of themselves as Christian or spiritual, in the west at least, are merely inconsistent materialists; their notion of spirituality an intellectual icing atop a materialist ground, which they take to be the reality of 'this world'.
I suppose the belief in ideas can itself be seen as a form of materialism, mental materialism. Reality condensed into supposed solid distinct substance, edifices created by the building blocks of language, which the mind then treats as living truth. Though come to think of it, materialism, ie the idea of materialism, is itself an instance of this mental materialism in the first place.
Above a self-portrait by Salvator Rosa, the words the austere artist is holding are "Be silent, unless what you have to say is better than silence." I'd always assumed from the nature of this painting that Rosa was Spanish- the harsh disdain for the world of the senses inimical to normal Italian sensibilities- but he was Venetian, though very influenced by, and a pupil of, the Spanish artist, Jusepe Ribera, who had settled in Venice.
As an afterthought, I wonder was Rosa in the habit of carrying around this sign in everyday life. The life and soul of the party, such a figure probably wasn't.