It probably seems a bit of a parodox that an atheist can do good deeds as well as a Christian. This can be explained because Christians claim that God is the source of all goodness.
When God created humanity he left what is called His divine imprint on us all. All societies have a natural goodness which was instilled in us by God and this can come out and has come out throughout the ages.
God's divine imprint gave us the capacity to develop a certain level of morality, but this morality was very crude and rudimentary compared to what Christ revealed to us during His time on earth.
This divine imprint is sometimes called The Natural Law. That is the basic principles of right and wrong are known to every human being even if those principles are distorted by human sinfulness. The Natural Law was instilled in us by God, since we are His creation. The Natural Law as Christians understand it has no real meaning without God.