To mainline Protestants, such sects as Seventh Day Adventists are "weird".
Trump doesn't have a filter, so he often says things that others think, but professional pols don't say.
OTOH...read posts from when Romney was first running for president, but quit, and see what posters had to say about Mormonism; for example!
Regardless of his reasons, it might indicate that Trump views that one's faith come from your parents or heritage, rather than each person making their own decision to believe that Jesus died on the cross to take the punishment for their sins and asking forgiveness so that they might have eternal life in Christ.
And why belittle Seventh Day Adventists and Carson's conversion? Trump is welcome to keep his faith private, but when he starts addressing the other candidate's faith and bringing a Bible to some events, it comes to the forefront.
You also have to wonder why he doesn't have someone in his campaign to tell him how all of this looks to many people.