Thomas Campbell was a Presbyterian minister important in the Second Great Awakening of the United States. Born in County Down, northern Ireland, he began a religious reform movement on the American frontier. He was joined in the work by his son Alexander Campbell. Their movement, known as the "Disciples of Christ", merged in 1832 with the similar movement led by Barton W. Stone to form what is now described as the American Restoration Movement (also known as the Stone-Campbell Restoration Movement).
The Campbell wing of the movement was launched when Thomas Campbell published the Declaration and Address of the Christian Association of Washington in 1809. In The Declaration and Address he set forth some of his convictions about the church of Jesus Christ, as he organized the Christian Association of Washington, in Washington County, Pennsylvania, not as a church but as an association of persons seeking to grow in faith.
On May 4, 1811, the Christian Association reconstituted itself as a congregationally governed church. With the building it constructed at Brush Run, Pennsylvania, it became known as Brush Run Church. When their study of the New Testament led the reformers to begin to practice baptism by immersion, the nearby Redstone Baptist Association invited Brush Run Church to join with them for the purpose of fellowship. The reformers agreed, provided that they would be "allowed to preach and to teach whatever they learned from the Scriptures." Thomas and his son Alexander worked within the Redstone Baptist Association during the period 1815 through 1824.
While both the Campbells and the Baptists shared practices of baptism by immersion and congregational polity, it was soon clear that the Campbells and their associates were not traditional Baptists. Within the Redstone Association, some of the Baptist leaders considered the differences intolerable when Alexander Campbell began publishing a journal, The Christian Baptist, which promoted reform. The Campbells anticipated the conflict and moved their membership to a congregation of the Mahoning Baptist Association in 1824.