Now this sign of the prophet Jonas he further explains here; (Mat_12:40) As Jonas was three days and three nights in the whale's belly, and then came out again safe and well, thus Christ shall be so long in the grave, and then shall rise again. [1.] The grave was to Christ as the belly of the fish was to Jonah; thither he was thrown, as a Ransom for lives ready to be lost in a storm; there he lay, as in the belly of hell (Jon_2:2), and seemed to be cast out of God's sight. [2.] He continued in the grave just as long as Jonah continued in the fish's belly, three days and three nights; not three whole days and nights: it is probable, Jonah did not lie so long in the whale's belly, but part of three natural days (nuchthe?merai, the Greeks called them); he was buried in the afternoon of the sixth day of the week, and rose again in the morning of the first day; it is a manner of speech very usual; see 1Ki_20:29; Est_4:16; Est_5:1; Luk_2:21. So long Jonah was a prisoner for his own sins, so long Christ was a Prisoner for ours. [3.] As Jonah in the whale's belly comforted himself with an assurance that yet he should look again toward God's holy temple (Jon_2:4), so Christ when he lay in the grave, is expressly said to rest in hope, as one assured he should not see corruption, Act_2:26, Act_2:27. [4.] As Jonah on the third day was discharged from his prison, and came to the land of the living again, from the congregation of the dead (for dead things are said to be formed from under the waters, Job_26:5), so Christ on the third day should return to life, and rise out of his grave to send abroad the gospel to the Gentiles.
3. Christ takes this occasion to represent the sad character and condition of that generation in which he lived, a generation that would not be reformed, and therefore could not but be ruined; and he gives them their character, as it would stand in the day of judgment, under the full discoveries and final sentences of that day. Persons and things now appear under false colours; characters and conditions are here changeable: if therefore we would make a right estimate, we must take our measures from the last judgment; things are really, what they are eternally. (Matthew Henry)