These are the scary words he speaks to the Pharisees. They dodge the comment and say, "Will he kill himself?" Jesus gives a caveat. He says, "unless you believe." For, if you believe on Jesus, then all of your sins will be forgiven.
In this passage, we see Jesus confront the religious leaders and offer them hope. He makes it clear that He was sent from the Father, and he tells us how we can have our sins forgiven.