John learns that the gods "were men -- they went a dark road, but they were men". He realizes how advanced they were, but also how they destroyed each other. This is significant information, considering who they have been revering for so long were just men, like them.
When he gets back, his dad asks him to tell him everything,and once he does, his father states, "Truth is a hard deer to hunt. If you eat too much truth at once, you may die of the truth." So, the reason his dad doesn't want him to share it all, at least at once, is that it is too much to bear. The people have been living a certain way, and believe certain things, and the information that John holds would change everything for them. So, his dad recommends that they "the truth should come little by little" to their people so that it doesn't crash their entire world around them. John agrees, stating that "Perhaps, in the old days, they ate knowledge too fast", which is what helped cause their downfall.
He learns that the gods were normal just like they were and his dad doesn't want him to tell them right off because it would hurt the people and their beliefs.