I haven't read Dabney's Stonewall bio, but I've read several others. he had become a devout calvinistic Christian by the time the war started, While he never spoke out for or against slavery, he did say it was found in the Bible without being condemned, and he was an utter believer in states' rights. He started several classes to teach blacks, both slave & free, how to read & write, encouraging them to read the Bible soon as they became proficient in reading. And he utterly detested any mistreatment of any slave, including overwork, neglect of medical attention, inadequate food or clothes, or beating/whipping as punishment, as well as rape & other crimes against them.
But his main cause for becoming a Confederate soldier was states' rights, & when Virginia seceded, he & Lee seceded with her.