They counter argue by saying that it is all God's test
If testing is the purpose, what is the logic behind creating children who are born severely retarded or in a vegetative state? They are not even able to grasp the concept. Why use torture techniques like horrible deceasesdiseases, deformed births, famine, floods, earth quakes etc. for testing? If a human used similar techniques, he would be called a sadist or a psychopath.
Also, if the so called God needs to test something, he is not all knowing. If he is not all knowing, he is not almighty either.
It's hard to convince both theists and atheists about Samsaric existences until they start meditating.
Once one starts to meditate, one can see that things don't just get created out of the blue. They come to be due to causes. It is only logical to think that birth also resulted in the same way.
One also sees how craving leads to clinging which in turn constructs life. It would be illogical to think that the pattern ends at death or a god will take over all of a sudden.