The reason that is such a bad saying is because as soon as you use the word "Christ", you have to define what you mean. A soon as you define it, you are making a creed.
After all, by "Christ" do you mean the enlightened guru of the Hindu mystics? Do you mean the leader of the Unification Church? Do you mean Satan's brother, as Mormonism teaches? Do you mean a heavenly power that came down on an ordinary man named Jesus of Nazareth when he was baptized, and then left him just before he died? Do you mean the respected prophet Isa of the Qu'ran?
"Oh no!," you say. "I mean the Christ of the Bible!"
"Why the Bible?", some cultist might immediately say. "Why not our book?"
Then, as soon as you start to defend the inspiration and authority of the Bible, as opposed to the Qu'ran, or the Book of Mormon, you are making more creed!
God's Word commands us to show ourselves to God as ones approved, workmen who do not need to be ashamed, handling accurately the word of truth (2 Timothy 2:15). To say "no creed but Christ" is the same as saying, "I refuse to study my Bible thoroughly or in-depth. I want nothing but vague, general notions of my faith. I do not want to be able to defend Christianity against cults. I am against being clear, and I am against having standards."
The Bible stands over all statements of faith, but the Bible also produces statements of faith. Christians throughout history have written statements of faith because they needed to defend the faith against cults, confusion, and controversy. So don't say, "No creed but Christ." It's a fundamentally anti-scriptural saying.