In 1824 Peck's preaching helped Illinois Governor Edward Coles defeat efforts to revise Illinois' constitution to permit slavery. Four years later, black Baptists in St. Louis sought to establish their own church, and with Peck's help they established the African Church of St. Louis (later renamed the First Baptist Church of St. Louis). Of the original 220 members, 200 were slaves. Peck ordained a young freeman, John Berry Meachum, as their pastor. When that church's members voted themselves out of existence, Peck helped establish the Second Baptist Church in 1833, serving as its interim minister three times in the 1840s.