Paul speaks of the Spirit sealing believers in three texts (2 Cor. 1:21–22; Eph. 1:13; 4:30). While interpreters often associate this sealing with the down payment or guarantee of the Spirit, they tend to associate the “seal” with the Spirit himself as either a seal of authority or a pledge for full salvation. Others identify the seal with confirmation. And there are many other positions besides these. My purpose here is not to dispute these conclusions but to point out how some Christians in the first centuries defined the seal of the Spirit as baptism itself. In this article, then,...