Correlation doesn't mean causation.
I could argue that art has gotten worse since we stopped using horses for transportation.
But actually, there is no correlation at all. Even if we only look at Europe, then ALL the art was produced in the [Renaissance](https://en.wikipedia.org/wiki/Renaissance). Which was from the 15th to the 16th century.
Therefore, the Christians who conquered Europe with the sword (i.e. so religious that they would fight for it) have produced fuck all. Christians have produced absolutely no art from the time they took over Europe until the 15th century (with the exception of Italy that started in the 13th).
Now explain to me why you think there was a millennia of Christianity in Europe and zero art.