sign - a typical representative in his own person of what was to befall them (Isa 20:3).
when this cometh - alluding probably to their taunt, as if God’s word spoken by His prophets would never come to pass. “Where is the word of the Lord? Let it come now” (Jer 17:15). When the prophecy is fulfilled, “ye shall know (to your cost) that I am the Lord,” who thereby show My power and fulfil My word spoken by My prophet (Joh 13:19; Joh 14:29).