Romans 6:4 says that baptism symbolizes our burial with Jesus—a sharing in Christ’s death in a way that allows us also to share in His resurrection life. It’s why we can now live life in a brand new way. When we are born again, we become connected to Jesus in such a way that we are actually joined to Him. And having become united with Him, we now enjoy all the benefits accomplished on the cross—just as if we had suffered the punishment of death ourselves. Baptism is a powerful symbol of that reality—joined to Christ, both in His death and in His resurrection.
The punishment for sin is separation from God—spiritual death. But when Jesus died on that cross, He became the appeasing sacrifice for our sins. He actually stepped in and substituted Himself in our place. Jesus suffered the agony of separation from His Father so that we don’t have to.