A.T. Robertson Commentary Romans 11:20

"Well; by their unbelief they were broken off, and thou standest by thy faith. Be not highminded, but fear:" — Romans 11:20 (ASV)

Well (καλως). Perhaps ironical, though Paul may simply admit the statement (cf. Mr 12:32) and show the Gentile his real situation.

By unbelief (τη απιστια)

--by faith (πιστε). Instrumental case with both contrasted words (by unbelief, by belief).