Claim:
"In the United States, atheists have lower divorce rates than any of the Christian sects."
Answer:
The first article on these topics I bounced into is called "The Effects of Religious Homogamy on Marital Satisfaction and Stability".
It doesn't discuss the actual divorce rates, but self-reported satisfaction in marriage and stability of marriage, so it is not exactly the same thing as you say, but anyway the closest I could find related to your claim.
The first relevant point there is that according to this article at least, you seem to making a mistatement. The pairs with no religion reported by far the least marital stability followed by the Baptist, then Catholic, then moderate Protestant, then conservative Protestant, then conservative Protestant, then liberal Protestant and finally nondenominational Protestant.
Second relevant point mentioned in the article is that religious affiliation, to which you refer, is not necessarily a very good indicator of a person's religiosity:
"Denominational affiliation may be the result of something completely unrelated to one's religious ideas, such as keeping the peace with one's parents, or trying to "fit in" better with one's associates."
That is why authors use rather two other measures of respondents religiosity: Church attendance and believe in the Bible. They find:
"Results indicated that respondents who attended church frequently, and those who report strong convictions about the utility of the Bible were more likely to report their marriage was very happy and had a low chance of dissolution."
