"If the elements in our atmosphere were even a few percentage points different, nearly every living thing would die."

If this comment is stating that God exists because if you were to change the percentage of elements in the atmosphere, it would would cause most life to die...this person must not know that the life alive today is only here because it evolved along side an atmosphere that it could live in. Life would never evolve in an environment in which it couldn't survive., The atmosphere in the beginning of our planet's formation 4-3.5 billion years ago determined what life could come about. 

Earth’s atmosphere used to have no free oxygen, and it was not until the first photosynthetic prokaryotic organisms produced O2 as a waste product 3.5 billion years ago that we started to build up oxygen in our atmosphere. We have also shown that oxygen then probably disappeared from the atmosphere around 1.9 billion years ago and it had little effect on the organisms. It was not until around the Cambrian 541 million years ago that oxygen was in higher percentages and it provided life with new possibilities like aerobic metabolism. Only about 2% oxygen level by volume is needed to make collagen  which is used by all animals and its only the large animals that need the higher levels. Large animals only developed after the oxygen increased in the Cambrian, thus my point that life evolves along side the atmosphere in which it can live in. The percentages of our atmosphere fit life today because we have been evolving with it for billions of years.

Since around 500 million years ago, O2 has been fluctuating between 15% and 30% of atmospheric volume and in the Carboniferous period 300 million years ago had up to 35% which probably contributed to the large size of bugs and amphibians. The life during this period evolved with the high O2 levels and wouldn't survive in today's oxygen %. 

If the percent of elements changed in our atmosphere changed so drastically that it caused mass extinctions  we would just be another extinction even for the books. Give it a few million years and a new set of organisms will have evolved to the new atmospheric levels and will have taken over.

Sources:

Berner, R. A. (Sep 1999). "Atmospheric oxygen over Phanerozoic time" (Free full text). Proceedings of the National Academy of Sciences of the United States of America 96 (20): 10955–10957.

Berner, R. A. (Sep 1999). "Atmospheric oxygen over Phanerozoic time"(Free full text). Proceedings of the National Academy of Sciences of the United States of America 96 (20): 10955–10957.