Importance of the Trinity?

The other day I was talking to a co-worker and I think we were talking about something related to faith (since I was going to seminary). His father is a Methodist minister and his wife and kids are Mormon. So he and his father had a discussion a long time ago about his kids being raised Mormon and his father asked him once, "You know they don't believe in the Trinity, right?" And he answered affirmatively. His father continued, "Doesn't that bother you?" And he essentially answered, "No, not really." To him, believing in the Trinity doesn't really seem to be a make-or-break thing, especially for the idea of salvation. And though I believe in the Trinity and believe there's probably a good reason to believe in the Trinity, I can't think of what that reason is.

In fact, I've always found doctrine specifics to be a bit elusive. Some things seem pretty clear to me (like the importance of the resurrection of Jesus), but I don't know that the Trinity is one of them. Perhaps this is part of why I'm looking forward to seminary; to understand such things. But I really wonder if not believing in the Trinity will cause me to cease being a follower of Jesus.