Baptismal Regeneration is the idea that water baptism is essential for salvation. It is one of the earliest heresies to enter the church as early as the 2nd century. The belief in baptismal regeneration was partly due to a misunderstanding of Jesus’ words in John 3:5. “Born of water” was seen by some as a reference to baptism. It also arose due to people confusing water baptism with baptism into the body of Christ (which is necessary for salvation).
The error of baptismal regeneration is that it requires man to do something in order to be saved. Water baptism is certainly an important result of salvation, but not a means to salvation.