John Chrysostom

Hence God gave the gift of prayer. But he does this, even though he does not need for us to ask, but that we might not grow indifferent from being saved without effort. For this reason, he said to Jeremiah, "Do not pray for this people, for I will not hear you," not wishing to stop his praying (for he earnestly longs for our salvation) but to terrify them. Seeing this, the prophet did not stop praying. So that you may see that God did not wish to turn Jeremiah from it, but to shame them that he said this, hear what it says. "Don't you see what they are doing?" Also when he says to the city, "Although you wash yourself with soda and use an abundance of soap, the stain of your guilt is still before me," it is not that he may cast them into despair that he so speaks, but that he may rouse them to repentance.