Many creationists (and, sadly, a few mislead skeptics) have the misconception that the so-called "Cambrian explosion" is a mystery to science, that it baffles scientists and is unexplained (and if something is unexplained, you know that arguments from ignorance will immediately follow.)
The so-called Cambrian explosion is the phenomenon found in the fossil record that multicellular life proliferated and diversified during the Cambrian period at an extraordinarily fast rate, and that before that period there's very little fossil evidence of any kind of life (although it's not completely non-existent.)
Note that when the term "fast rate" is used in this context, it refers to "fast" in the geological sense. In other words, when normally such amount of diversification requires hundreds of millions of years, here we are talking about just a few tens of millions of years. While that's "very fast" in a geological timescale, that's still a humongous amount of time. (People have great difficulties in understanding the magnitude of time that tens of millions of years encompass, because it's so far removed from our everyday experience.)
Creationists and other people who do not understand evolution at all may be easily misled by such fancy words as "Cambrian explosion" and "extraordinarily fast rate of evolution", and believe that we are dealing with something that baffles scientists, who struggle for an explanation. But that's not the case at all.
In fact, if you understand how evolution works, this is not a mystery at all. It's simply the case that when successful multicellular organisms first developed, they were so successful and superior at surviving, and they found such a huge, huge niche where they could freely live in, that they successfully reproduced and diversified with little hindrance for a very long time. (In economic terms, they found an enormous market with no significant competition.) The world was stock full of nutrients (by that time diverse single-cell organisms had spread everywhere) and there was little to no competition for them.
This is also the time when multicellular organisms started to develop harder tissue that could fossilize. Prior to that the vast majority of organisms were soft-tissued, or even unicellular. The almost complete lack of fossils prior to the Cambrian is no mystery at all. There was simply nothing to that would leave fossils behind, because most organisms were soft-tissued. (That's not to say that no fossils at all exist in the pre-Cambrian layers. They do. It's simply that they are much, much rarer than in the upper layers. It requires much more special circumstances for soft-tissued organisms to fossilize, not to talk about unicellular ones.)
Of course creationists willfully ignore all this, in order to retain one argument more in their repertoire. And Christians are supposed to keep honesty in a very high regard...