Does a post religious world mean no God or gods? God no. The concept of God, or gods, or spirits, superstitions, etc have been with the human race forever and I imagine they will exist for quite a long time into the future. Irrational beliefs will likely never fully go away. Post religious to me means that people would not look to religious leaders or religious text as a way of determining moral matters. Instead, they would use their inherent knowledge of ethics, along with ethical understanding (either taught and/or self derived) to determine moral matters. I use the term morality, because it describes items culturally designated as forms of sacred values where ethical understanding is required to derive the proper action. Overtime, the moral framework should be properly balanced with ethical understanding.
The only way cultures in the past have been able to be integrated was through religious tolerance. Tolerance of one’s “strange” customs is one thing, adhering to another’s cultural rules governing moral matters is something that has always created friction and will continue to do so in the future. That being said, if “we the people” of this world all had a proper understanding of ethics, it would allow all cultures, regardless of their religious traditions and interpretations, to have a common foundation from which disputes could be argued and settled.
Does knowing right from wrong lead to a world were everyone does the right thing? Highly doubtful. Those intent on doing harm or those intent on doing good that unintentionally causes harm will always exist. A proper understanding of ethics however leaves those causing harm or treating others unfairly little room to hide. When you take away the moral framework of the world (Authority/Respect, In-group/Loyalty, Purity/Sanctity) all the bullshit fades away…things like God and Country (group affiliations; including ethnicity), devilish memes (Authority in the form of God’s will; or Agenda-style, “mission focus” memes, etc), and non-sense conventionally defined elements of purity/sanctity (Respect of the Sabbath, cultural subjugation of women based on ideas of purity, etc); all of it fades away and becomes obsolete, relicts of a past time.