John Wesley on Matthew 12:39

An adulterous generation - Whose heart wanders from God, though they profess him to be their husband. Such adulterers are all those who love the world, and all who seek the friendship of it. Seeketh a sign - After all they have had already, which were abundantly sufficient to convince them, had not their hearts been estranged from God, and consequently averse to the truth. The sign of Jonah - Who was herein a type of Christ.