They believe baptism effects a change in the soul of the convert. Fundamentalists don't believe that. They think baptism produces no change in the soul. Rather, it's an outward symbol of conversion. What separates Mormons from Catholics on this issue is the understanding of original sin and it's relationship with baptism.
Catholics believe in original sin.
For Mormons, there is no sacramental cleansing of original sin because there is no original sin that needs to be cleansed. That's why Mormons don't baptize infants. To help Mormons understand the importance of baptism, you must demonstrate the fact that all men are, until baptism, out of God's friendship due to Adam's sin. 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.