“We become what we behold. We shape our tools and then our tools shape us.”
—Father John Culkin
According to McLuhan Galaxy:

[This quote] is often mistakenly attributed to Marshall McLuhan. It does NOT appear in “Understanding Media”, as Wilson Miner confidently asserts in the presentation below, indeed it does not appear in any published work by McLuhan at all. The quote was actually written by Father John Culkin, SJ, a Professor of Communication at Fordham University in New York and friend of McLuhan. But though the quote is Culkin’s, I would argue that the idea is McLuhan’s, as it comes up in an article by Culkin about McLuhan: Culkin, J.M. (1967, March 18). A schoolman’s guide to Marshall McLuhan. Saturday Review, pp. 51-53, 71-72. The idea presented in the quote is entirely consistent with McLuhan’s thinking on technology in general.