Not a natural public speaker, Rankin worked hard while at Jefferson County Presbyterian Church simply to deliver an effective sermon. Within a few months, however, despite Tennessee's status as a slave state, he summoned the courage to speak against "all forms of oppression" and then, specifically, slavery. He was one of the founders of the Tennessee Manumission Society, in 1815. He was shocked when his elders responded by telling him that he should consider leaving Tennessee if he intended ever to oppose slavery from the pulpit again. He knew that his faith would not allow him to keep his views to himself, so he decided in 1817 to move his family to the town of Ripley, across the Ohio River in the free state of Ohio, where he had heard from family members that a number of anti-slavery Virginians had settled.