A.T. Robertson Commentary Acts 13:41

"Behold, ye despisers, and wonder, and perish; For I work a work in your days, A work which ye shall in no wise believe, if one declare it unto you." — Acts 13:41 (ASV)

Ye despisers (ο καταφρονητα). Not in the Hebrew, but in the LXX. It is pertinent for Paul's purpose.

Perish (αφανισθητε). Or vanish away. First aorist passive imperative. Added by the LXX to the Hebrew.

If one declare it unto you (εαν τις εκδιηγητα υμιν). Condition of third class with present middle subjunctive, if one keep on outlining (double compound, εκ-δι-ηγεομα) it unto you. Paul has hurled a thunderbolt at the close.