Imagine a world with many gods that have been in constant battle with each other. However, the gods are exactly equal in strength, the very nature of godhood enforces a sort of balance in their strength, meaning neither side has been able to defeat the other. The gods have been in a stalemate for a very long time.
This war is not just metaphysical. Actual physical entities exist, roughly equivalent to your demons and angels (though the gods are not divided by a clear good or evil division that would imply). These entities were originally created by their respective gods and are innately predisposed to following that gods demands, with the more powerful able to move between spiritual and physical planes.
The gods themselves also have a physical body that represents a portion of their strength and could in theory die if overwhelmed by enough angels/demons, though they are drastically stronger then your average demon. War has been fought using these entities and by the gods themselves on both physical and metaphysical planes.
Then humans appeared on earth, which is a very recent development compared to the long life of gods. Humans were not (directly) created by any specific god, indirectly human evolution was made possible by the power of the gods being so abundant that humans soaked up a small portion of the power. This was not an intentional action by the gods.
As a side effect of how humanity came about they are the first sapient creature that is not bound to any specific god's source of power. They are not forced by their very nature to side with one god or another, and have free reign to pick who they ally with. This makes them an outlier, and a possible kingmaker. If the majority of humanity sided with one set of gods their extra strength would be enough to give the gods an edge in the battle, breaking the tie and allowing one side to finally defeat the other.
The gods are now spending much of their time trying to court humanity, to draw humans to their side in hopes of tilting the balance in their favor. They still war, but the warring has slowed some as they focus on gaining allies within humanity.
My main question is: How can humans be a believable deciding factor in the gods war while still being appropriately limited in strength? That is to say most humans are tiny and pathetic compared to the power of a God. A trained human may be able to deal with the weakest of the gods' followers (think imps and the like), but most humans wouldn't stand a chance against your average demon, and gods have a vast number of followers. Some humans, the greatest of them, will grow to be strong enough to defeat demons, but few will grow strong enough to defeat even a moderate strength demon in battle. Even the rare few humans able to take on the stronger demons are still tiny in numbers compared to how many of these demons gods already command, making them seem minor compared to existing gods' strength.
Many humans never take sides in the war, and of those that do take sides many end up on opposite sides, counteracting each other. The final result seems to be that humanity is a minor resource compared to that which already follows the gods, making one wonder if the energy spent courting humanity is worth the minor extra strength they offer. Thus I'm trying to figure out how to better explain why gods care enough to be trying so hard to court humanity; and generally how to make the heroes of such a world have more impact.
Gods do not gain power through belief or worship; they want humans to fight directly in the war not be some metaphysical battery pack. Gods can bestow power on humans, which is efficient enough to be worth doing, but I need humans who are not gifted power from a god to be at least close to the strength of those that are, meaning that there is an effective cap on how much strength humans gain from the gods, and it's not enough to make them unstoppable war machines by itself.
This is your general sword & sorcery kind of story, so the classic roughly-medieval technology level for humans should be assumed. Magic exists, independent of gods' power, but it has upper bounds on its strength as well.
What else can be done to increase the importance of humans in the gods' war?