Charles Ellicott Commentary Matthew 12:39

"But he answered and said unto them, An evil and adulterous generation seeketh after a sign; and there shall no sign be given it but the sign of Jonah the prophet:" — Matthew 12:39 (ASV)

An evil and adulterous generation — The true relationship between Israel and Jehovah had been represented by the prophets as that of a wife to her husband (Jeremiah 3; Ezekiel 16:23; Hosea 1:2). The adulterous generation was therefore one that was unfaithful to its Lord, demanding a sign instead of finding sufficient proof of faithfulness and love in what He had already done.

There shall no sign be given to it — These words seem at first to place our Lord’s miracles of healing outside the category of signs, and yet it was to these that He referred the messengers of John the Baptist as proof that the Christ had indeed come (Matthew 11:5). The statement must, however, be interpreted by its context. One sign, and only one, of the kind they demanded would be given to those for whom the other marks of Messiahship were insufficient, and that would be the sign of the prophet Jonas.