{Because it is written} (dioti gegraptai). "Because
(dioti stronger than hoti below) it stands written" (regular
formula for O.T. quotation, perfect passive indicative of
graph(935c)). The quotation is from  Le 11:44  19:2  20:7 .
Reenforced by Jesus in  Mt 5:48 . The future esesthe here is
volitive like an imperative.