People who do not believe in God also do good things, without faith. How do you understand that?

Luther:

They indeed do that, but they do it like children who only do what the teacher says, because they are afraid of punishment or they want a reward. Meanwhile they mostly dislike the teacher and they actually would rather want be free and to do what they want. They do not do the good out of love and out of free will, but because they must.

That this is so can be seen in a country where the government ceases to exist. When the system of penalty and reward expires, you shall see that chaos breaks out and everyone does what he wants. God still keeps sinners in check by the government.