Audacity wrote: NOT to
define it, but to
make your case for its truth.
Yes, that is what I meant. Mistyped.
but per the follow up to the OP, the definition of "creationism" is "The religious contention that all species of life were put on earth as is: no species evolved from any other species." That strikes me as self-contradictory. I'm being asked to show why I believe that God created life without using evolution, but I'm not allowed to say that God didn't use evolution. :-/
No, you're not being asked any such thing. If you believe that all species of life were put on earth as is: no species evolved from any other species, then that's all you have to address. Forget any alternatives, such as evolution, exist, and simply stick to your belief and make your case for it. Now, if you have to bring evolution or its principles into your case then you've failed, pure and simple.
You missed my point that time. Suppose someone believes that God did not use evolution. As far as I'm reading you, when you say to make our case for whatever version of non-evolutionary creationism we hold to, you want us to offer some sort of reasonable defense as to why we (ought to) believe it. But then you say "forget any alternatives."
How is someone supposed to explain why they believe something if the only way logically possible to offer such an explanation is to show that the disjunctive is impossible.
That's what I was getting at in my overall posts. There are two, and only two, ways to answer your question. The positive case can only be made by appeal to Scripture. The negative case can only be made my showing the evolutionary disjunctive fails. Therefore, unless you are prepared to accept the witness of Scripture, your question by nature unanswerable. Let me give you a practical example as to why. Suppose I say something like,
"One prediction of creationism would be that we would have all the phyla of life coming into existence fully formed. We see that in the Cambrian explosion. Therefore, we have reason to believe creationism."
Now you would have two responses to that. First, you would object to the phrase "fully formed," since what that means is, "without any intermediary forms preceding them." But that phrase is just another way to talk about evolution. This is sufficient, though, to show that it is impossible to defend the position while forgetting the alternatives.
But your second response would be to give a scientific (which is to say, an evolutionary) explanation of the Cambrian Explosion. Now, if we have to forget evolution (or any other alternatives), what are we to say in response? And if we know that you are going to raise the scientific explanation as a plausible alternative, then why not include that in our original argument. After all, the argument would actually be something like:
1. Creationism predicts that all phyla should appear fully formed "suddenly" in the fossil record;
2. The Cambrian Explosion is the sudden appearance of all fully formed phyla in the fossil record;
3. If there is only two logical possible explanations for a piece of data, and one is shown to be unable to explain the data, then the remaining ought to be accepted as true;
4. The only other possible explanation for the Cambrian Explosion is evolution
5. But evolution cannot explain the Cambrian Explosion
6. Therefore, Creationism is both confirmed in terms of prediction and ought to be held as true, given that it is the only remaining explanation of the data
So that would be the kind of argument someone might make. But the entire second half of that is with reference to evolution. I don't even know how you would go about providing any argument in favor of creationism without using some form similar to this.
That's why I said I don't know what you're asking for.
, but it's not for the fault of anyone's lack of response. 
Carbon is the building block of life, so evidently we are all carbon-based.