The only reason that anyone can be moral is because God’s law is stamped on their hearts:

for when Gentiles, who do not have the law, by nature do the things in the law, these, although not having the law, are a law to themselves, who show the work of the law written in their hearts, their conscience also bearing witness, and between themselves their thoughts accusing or else excusing them.(Romans 2:14–15)

This moral law is often masked or darkened because of our sin nature, but the remnant of God’s law is still visible on the hearts of all. Just as everyone knows—whether they admit it or not—that God exists (Romans 1:19), so do they also know God’s moral law. That’s why even those who reject God can lead “moral” lives—God’s law is written on their hearts. This is also why everyone is “without excuse” (Romans 1:20). When atheists participate in some moral things—such as wearing clothes or telling the truth—they are proving the truth of God’s Word and confirming His existence.