Jesus answered them, “Destroy this temple, and in three days I will raise it up.” John 2:19
Jesus has just finished clearing the Temple of the money changers and people who had defiled such a holy place. The religious leaders confronted him and demanded that he give them a sign that would prove his authority to cause such a disturbance. They wanted proof that he was a true prophet. His answer only served to confuse them even though it really pointed to the most powerful sign of his authority.
Jesus said that if they destroyed this temple, he would raise it up in three days. understandably the religious leaders thought he was talking about the physical temple that he has just cleared out. It was nonsense to them. First of all they would never tear down the Temple. It was a sacred place. Secondly, there is no way that Jesus could rebuild something in three days that took years and hundreds of people to build in the first place.
With hindsight we can see that Jesus was actually talking about something far more miraculous. He was talking about the temple of his physical body that the religious leaders would become very willing to destroy. If they were to destroy his body Jesus was claiming to be able to raise it up in three days. Bringing himself back from the dead after three days was a far more powerful miracle that being able to stack a bunch of stones back on top of one another.
The resurrection that Jesus predicted is central to the Gospel and a powerful vindication of Jesus. It is the ultimate demonstration of his right to not only do what he did in the Temple that day, but his right to claim allegiance of all humanity to him as the King of Kings and Lord of Lords. The fact that the Father raised Jesus from the dead following the crucifixion should be seen as proof that he alone is the messiah and the one to whom we should bend the knee and worship.