A.T. Robertson Commentary Acts 28:26

"saying, Go thou unto this people, and say, By hearing ye shall hear, and shall in no wise understand; And seeing ye shall see, and shall in no wise perceive:" — Acts 28:26 (ASV)

Say (ειπον). Second aorist active imperative instead of the old form ειπε. The quotation is from Isa 6:9,10. This very passage is quoted by Jesus (Matthew 13:14,15; Luke 8:10) in explanation of his use of parables and in Joh 12:40 the very point made by Paul here, "the disbelief of the Jews in Jesus" (Page). See on Matthew for discussion of the language used. Here the first time ("go to this people and say") does not occur in Matthew. It is a solemn dirge of the doom of the Jews for their rejection of the Messiah foreseen so long ago by Isaiah.