From the Artscroll Chumash’s commentary in last week’s parashah for Vayikra 5:21:
The verse states that a person sins by being treacherous to G-d, and then it goes on to discuss cases of people cheating in business and the like. The Sages comment homiletically that one who trespasses against G-d will eventually deal falsely with his fellows. The Tosefta quotes R’Reuven who said that the most hateful person is one who denies G-d, for once a person denies the authority of the Lawgiver, he can easily violate all the norms of morality (Tosefta Shevuos 3:5). Atheism is the forerunner of morality’s destruction, because without the norms decreed by G-d, man can easily rationalize every manner of crime. Once man arrogates to himself to decide what is right and what is wrong, he can descend the amoral abyss, as contemporary history demonstrates (R’Yosef Dov Soloveitchik).