"There must have been a first cause, so there is a god."

 

"God must exist because I can show abiogenesis is impossible."

 

This is a problem you can point out with an argument that makes the argument pointless before it even gets started. If somone is claiming to be able to prove something by means of an argument, and the thing they wish to prove does not follow from the conclusion of that argument, then it doesn't matter whether the argument is valid or not. Either way, they cannot prove their point.

 

In the above first example, used in such things as the Kalam cosmological argument, theists try and prove god exists by establishing a "first cause". I can dismiss this argument as irrelevant, because its conclusion does not reach what the theists wants to establish. A "first cause" isn't even necessarily any sort of intelligent life, let alone a god. So even if their argument is completely valid, they have not demonstrated what they have claimed. By pointing this out, I save us both the time of hashing out an argument which is of no relevance.

 

In the second example, it doesn't matter whether or not the theist can successfully argue that abiogenesis is impossible, because it does not logically lead to the existence of a god either. All it would show is that another explanation for life is needed, it doesn't mean they get to make up whatever explanation they like the most.

 

In these cases, the theist needs to go away and come up with an argument that at least reaches the end result they want.