Last Sunday in my Sunday school class, the topic that came up here on the blog recently also came up there: the difference between trusting in divine grace, and trusting in one's understanding of divine grace.
One can apply the same point to the creeds. There is a difference between believing in God, and believing a creed, even if the creed begins with “I believe in God.”
Hence the title of the post. For some, it is not enough to believe in God. One must believe in “I believe in God…”