Atheists can’t be good people?
There aren’t any good people.
Atheists can be good people, just like the rest of us. They are born with a conscience like everyone else. But they have no solid philosophical reason or basis to believe their conscience or in an absolute value system. Philosophically and logically, all they can say is that they prefer one set of behavior over another, and to take a pragmatic approach. But a pragmatic approach doesn't work in those times when we can choose to do things when we know that we won't get caught. So atheists, who act good, end up acting in an inconsistent manner.
Theists, especially Christians, not only have a conscience, but also an external objective standard and a logical philosophical basis for acting good. Unfortunately, we, like everyone else, usually do not live up to those external standards. We also then can recognize our lack of goodness (sin), and be thankful for God's mercy that we do not at all deserve.