Initially they won't agree with you because they've been taught that in a way it was actually a good thing that Adam sinned. Although Scriptures abundantly show that men will be punished for their own sins and not for Adam's transgression, the common view is that all men are tainted with sin and denied blessings because of Adam's fall. He ridicules Catholic teaching, but doesn't offer the reader any evidence that would support the Mormon position. The conflict lies in Mormonism's teaching on free will. Does To explain Catholic teaching on baptism you need to demonstrate the reality of original sin, its effects on the souls of all people, and the necessity of baptism for its removal.
Several things happen at baptism.
This removal is accompanied by an infusion of sanctifying grace, which makes the soul spiritually alive. The soul receives an indelible character that irrevocably identifies it as a member of the heavenly family. Also, all punishment due to pre-baptismal actual sins is completely remitted. Our chance to become heirs with Christ comes here on earth. Once we've died, there is no chance to be baptized.