I offer this excerpt from D. Newell Williams, Barton Stone: A Spiritual Biography (St. Louis: Chalice Press, 2000), 165, which is what Alexander Campbell believed made one a Christian.
To be a Christian, [Campbell] asserted, was to believe that Jesus was the Messiah, upon the testimony of the apostles, and to be baptized, in accord with the apostolic practice, in the name of the Father, and of the Son, and of the Holy Spirit. He declared that confession that Jesus was the Christ, followed by baptism, was all that was necessary for the union of Christians.
Nowadays we draw the lines beyond the basics of salvation and allow one’s Christianity to be defined by doctrinal stances, worship practices, and various other means. Could we agree with Campbell that this is what makes one a Christian?