One might ask, "aren't all Christians (and in fact almost all religious people) creationists? After all, they believe that the universe was purposefully created by some kind of supernatural deity."
I suppose that technically speaking, in a very broad sense of the word, that may be so. However, I would distinguish (and I doubt I'm the only one that does this) between simply believing that the universe was created by some being, and actual "creationism".
Most Christians (and believers of several other religions) do believe that the universe was created by God, but they don't make almost any other claims about it, nor do they claim that, for example, it can be scientifically proven.
And that's what makes them different from "creationists". That term is usually used to refer to a set of beliefs that goes well beyond a mere "God created the universe". It's a, oftentimes quite large, set of beliefs regarding that act of creation, characterized by the assertion that we have actual scientifically observable and testable evidence for it, and/or that supernatural creation can be scientifically inferred. In other words, it's not simply a vague "I believe God created the universe even though I don't know any details about it", and goes far into a "I believe it, and it can be scientifically proven." That is what's usually meant by "creationism".
A particularly egregious and extreme version of creationism is the so-called "young-Earth creationism", which is used to distinguish it from "old-Earth creationism". The latter tends to at least try to stay within the realm of scientific facts and plausibility, while the former is not only full of absolute pseudoscience but is also a full-on bona fide delusional conspiracy theory (that believes that all scientists are in collusion and conspiring against Christianity by telling things they know are lies.)
Anyway, to the actual topic at hand.
One thing that especially young-Earth creationists love to claim, in order to prove that the universe was created by God, is that there exists a scientific law called the "Law of Cause and Effect". Many young-Earth creationist preachers state that as if it was a well-established fact, and something that's just one among any of the other scientific laws that exist.
To the layman it actually sounds plausible. After all, don't all effects have a cause behind them? It sounds like it just is one of the scientific laws of physics.
It might be a bit surprising to many, but there actually is no such law of physics. It might be that, maybe, some physicists might have sometimes suggested something akin to that, but the fact is that there is no commonly and widely accepted such law. You will not find it in any physics textbook or academic paper, including those that meticulously list different laws of physics.
The closest thing we have to it (and which is very likely where the creationists got the idea from) is Newton's Third Law of Motion, which states: "For every action, there is an equal and opposite reaction."
That is, indeed, quite close to the idea of "cause-and-effect", but it's not exactly that. Newton's Third Law is not talking about all possible effects, only those involving object interactions that have mass and move with respect to each other. (And the reason for the Third Law is conservation of momentum, which is in itself one of the most important laws of physics.)
The crucial and critical thing is that creationists have taken that Third Law of motion and generalized to all possible "effects". Such as, for example, the universe beginning to exist. That's not what the Third Law deals with.
Even more crucially, many creationists don't stop there, but actually have come up with their own definition of "the law of cause-and-effect". One such definition is: "For every material effect we see, there is a cause that came before it, or was simultaneous to it, and that is greater than it."
That definition is 100% the invention of creationists (especially of the young-Earth kind). You will not find that in any scientific textbook, paper or publication. It doesn't even make any sense scientifically (especially the last part.)
The creationist preachers who claim that such a "law of physics" exists, and that that is its definition, heavily implying that it's a definition that scientists came up with and is widely accepted in science, are being deceitful. In other words, they are lying.
