Atheist
Can't people be good without God?
Individual non-believers can certainly act morally, since the natural law is written on every human heart, but the Church argues that without God there is no ultimate, objective foundation that can ground moral obligation as truly binding rather than a matter of preference or evolutionary instinct.
Catholic teaching
The Church teaches that the natural moral law is accessible to reason and written on every heart, so unbelievers can genuinely perceive and do good; yet ultimately moral goodness finds its coherent source and end in God, who is Goodness itself, without which moral realism becomes difficult to ground.
Biblical foundation
Paul teaches that Gentiles 'who do not have the law' can nonetheless 'do by nature what the law requires,' showing the law 'written on their hearts' (Rom 2:14-15) — an affirmation that goodness is accessible outside explicit faith, even while its ultimate ground remains God the Creator.
Scripture references
Historical evidence
Vatican II's Gaudium et Spes explicitly affirmed that atheists who sincerely seek truth and follow their conscience can attain eternal salvation, an official acknowledgment that goodness and even grace are not restricted to explicit believers.
The Church Fathers
“Those who lived reasonably are Christians, even though they have been thought atheists.”