An outward sign of an inward change...
We baptize believers (i.e. after their conversion) as a vivid picture of what had happened to them spiritually when they repented of their sin and were saved as a result of believing the Gospel. Baptism is done by full immersion in water as a tangible, physical representation of this spiritual reality, and it acts as an adequate symbol of Jesus’ death, burial and resurrection.
For an in-depth explanation of baptism, download our official position on the subject.