Just before he makes this statement, John has been to the city of the gods, which was really just the ruins of a great and advanced civilization. He discovered many books, learned truths about these people and their destruction, and had his eyes opened to the many technological advances that they possessed. He realizes that the people of this civilization had life very easy and were extremely comfortable; he also realizes that they had great truths and wisdom. In their books, it revealed that they understood so much, and had so much knowledge at their fingertips. Knowledge is something that it is evident from the beginning of the story that John craved. So, he comes back to his tribe, and realizes just how far from all of that they are; they are living so differently, and have regressed in so many ways.
John wants to build society again, to make those same advances in technology and in wisdom. He wants to have all of the knowledge that the people in the city of the gods had. He realizes that to do that, they must start building again--not just literally, but building their knowledge base, building on their perceptions, and opening their eyes and minds to the possibilities that exist.
John's real quest isn't only just to build, but to build without making the same mistakes as their predecessors. His father warns him that too much knowledge all at once can be harmful, and John agrees. He realizes that even though these people had all the knowledge and advancements that they could want, they did not use it wisely. They "ate the truth too fast." Too much power that isn't tempered by wisdom, frugality and maturity only leads to destruction. So, he wants to build again, but slowly, the right way, so that they can have all of the good things of the old civilization, without the destruction that ended it.
I hope that those thoughts helped; good luck!
First of all it is a cool statement to end a story with, second of all they just realized that they are living in like 50 years behind in technology and stuff and they need to catch up to get get some modern conveniences.