The development of religion and government counted as an agent of conquest because it helps ensure that a society will have the soldiers and the resources needed to fight other societies. This gives a society that has organized religion and government advantages over a society that does not.
A society is likely to conquer another society if it has more soldiers, more weapons and more of other sorts of resources. A government makes this more likely to happen. A government can require all of its people to pay taxes. A government can use that tax money to pay soldiers and to buy weapons and equipment. A government can even force people to fight as soldiers.
While we may not think this would be true, organized religion can help with parts of this process. Organized religion can convince people that they are fighting for God (or the gods) when they fight for their country. It can help to motivate people to want to fight because it makes them think their fighting is for a higher purpose. It can convince them that they will be rewarded in the afterlife if they die fighting.
In these ways, governments and religions make it more likely that a society will have the soldiers and resources it needs to conquer. This makes these things agents of conquest.