[εδραιωμα “pillar”] can equally mean “foundation” (in the sense of the source of truth) or “support” (in the sense of defender or protector of truth). (Eric D. Svendsen, Evangelical Answers: A Critique of Current Roman Catholic Apologists [Lindenhurst, N.Y.: Reformation Press, 1999], 22)
Perhaps, though, the Greek construction here is best taken as a hendiadys (“one through two”), which means that the second term is a synonym for the first (“the church, which is the pillar—that is, defensive wall—of the truth”). (Ibid., 196 n. 41)