“I contend that we are both atheists. I just believe in one fewer god than you do. When you understand why you dismiss all the other possible gods, you will understand why I dismiss yours.”
– Stephen Roberts
I've commented on this atheist trope before, but I'll take another whack at it. You still have atheists who compare belief in Christian theism to Greek and Nordic mythology, Hindu polytheism, ancient Near Eastern mythology, &c.
1. At best, pagan gods are humanoid beings with superhuman powers. And some pagan gods are theriomorphic gods, or hybrid gods (i.e. half human/half beast) or deified natural forces.
However, standard theistic proofs aren't even applicable to gods like that. If some version of the ontological argument is sound, it only applies to a necessary being. But pagan gods aren't necessary beings.
If some version of the cosmological argument is sound, it only applies to an ex nihilo Creator of the universe (or multiverse). But pagan gods aren't creators in that absolute sense.
Likewise, the cosmological argument from contingency presumes that God is not contingent. Yet the pagan gods are contingent. Even if the argument is sound, it doesn't apply to pagan gods.
If some version of the teleological argument is sound, it requires a God with vast knowledge and magisterial control over the variables. But pagan gods lack those attributes.
Likewise, you have theistic arguments based on God as the necessary support for abstract objects (e.g. numbers, possible worlds). But pagan gods can't fill that role–even in principle.
Pagan gods aren't ultimate beings, but derivative beings. Finite beings.
Not only is there no evidence for the existence of pagan gods, but there's evidence that humanoid, physical beings can't have the superhuman powers which pagan mythology ascribes to their gods. Rather, we'd expect them to be subject to the same natural limitations of any physical being.
2. In classical theism, God cannot be affected by the world. God subsists outside of space and time. In Calvinism, God knows the future because God planned the future.
Pagan gods are not analogous to classical theism or Reformed theism. They are fundamentally different kinds of beings. By the same token, the spirits of heathen animism aren't analogous to Christian theism. So the reason to reject their existence can't be the same as reasons to reject the deity of classical theism or Reformed theism.
3. Christians affirm the God of OT Judaism. Christians reject the God of Islamic theism because Muhummad was demonstrably a false prophet; because the veracity of Islam is dependant on the veracity of the Judeo-Christian tradition–and yet that standard of comparison invalidates Islam.