Luke 11:30

The sign of Jonah is not further explained as in Mt. (Matthew 12:40), and it might seem that the meaning intended was that Jonah, as a prophet and through his preaching, was a sign to the Ninevites, and that in like manner so was Jesus to His generation. But in reference to Jesus Lk. does not say “is” but “shall be,” ἔσται, as if something else than Christ's ministry, something future in His experience, was the sign. Something is obscurely hinted at which is not further explained, as if to say: wait and you will get your sign.