They would not form religion yet; they might have supernatural beliefs, but they wouldn't have anything very organized that you could call a religion. The majority of their time will be spent gathering resources; assuming they are not into agriculture yet, they will have spare time to develop better tools to help them gather, which will further increase their spare time. They are unlikely to split up and battle each other as they will have nothing to really conquer, as food is abundant and wealth is largely nonexistent; it's not until food begins to run out--the breaking point between peace and chaos--that the stronger individuals will begin to dominate by demanding the lesser individuals follow them; this is a survival mechanism, wherein the stronger individual can use the energy of the lesser individuals to ensure his own survival, though he may insist it is for the survival of them all. The tribes battle and take each other's food; the more powerful tribes, who are better at surviving this way, are the ones which will continue to procreate. This continues until all the food is gone and everyone dies from cannibalism or starvation.
Now, at some point in time, or in a 2nd scenario considering they understand agriculture, this dark ending changes course and the majority of the tribe's time is spent ensuring their farms can continue to produce food. They tend to war less as they have the basic necessities of life and have all their energy focused on sustenance; as agricultural methods improve, e.g. animal assistance, fertilization and farming tools, food quantities increase, the population grows, people can focus on doing things besides hunting or farming and have greater opportunities to, again, improve their living conditions. Meanwhile, the stronger individuals, with the help of straggler tribe members--those who do not contribute a substantial amount--may insist that the work individuals are producing should be shared equally among the tribe members, of which failure to do so may result in ostracism or physical harm. If successful, individuals tend to give in only a bare minimum of their effort, no savings can accrue to allow free time for innovation, and the tribe remains as a series of dirt huts for the next millennia; this is known as egalitarianism. If unsuccessful, the stragglers may be supported by their individual families or pass away, while those who can save have better opportunities for improvement, and figure out more effective, efficient ways to get the most out of what they have to work with; better farming methods, more comfortable living conditions, sturdier houses, better waste disposal, time for art, music and developing language, formal and organized religion, on and on.
Two tribes who do not practice egalitarianism will realize the other has great new advancements in technology and different things to offer; they participate in trade and improve mutually. They do this as they have a fundamental understanding of human rights, including civil and property, as they were able to understand why the egalitarian route was detrimental longterm for their survival. Any tribe which does practice egalitarianism may attempt to conquer the tribes which do not, under the assumption that they, tribe A, deserves the fruits of the labor of non-egalitarian tribes B and C; tribe A does this because it is running seriously low on resources, after having consumed most of them and demoralized their working class, and if they don't conquer anyone, they will soon perish. If unsuccessful, tribe A will disappear, and so have a huge life-or-death incentive to go to war. If successful, tribe A's strongest will assume control over tribes B and C and, if he didn't already, will come up with a name for himself along the lines of "king" or "president". Tribe a continues to eat the resources of tribes B and C until they, too, become close to death, and thus will attempt to conquer and assimilate the next tribe. Meanwhile, individuals who do not agree with tribe A's egalitarian practices, continue to attempt to remove themselves from their society so they can continue working and saving so they can improve; tribe A's king insists that everyone must participate for their society to work, and threatens "traitors" with death if they do so. There, the king, who does not have to work, comes up with a plot: if he codifies his desire into "law" and claims they are given to him by divine sovereignty, he can have something to point to when someone disagrees with him. Further, the king insists he requires more equality than the others so he can perform his kingly duties; the amount he needs is at his discretion and failure to comply will result in punishment.
Anyway, as advancements occur, albeit very slowly, and populations grow, tribe A conquers more and more tribes until there are but a few very large tribes who practiced the same deeds as tribe A; we can now call these tribes, kingdom A, kingdom B, kingdom C etc. The kingdoms fight over resources and power to get more resources until they become the size of nation-states (how big is this island anyway?), technology has finally advanced to the point that people can stop working at farms and do other things with their lives, political thinking leads to concepts like capitalism, socialism, anarchism, fascism, which are all just rebrandings of events which occurred long ago but nobody remembers as too many generations have passed and nobody has the time to stop and study anyway until just recently; the nation states get into an arms race to rid the world of one another once and for all but eventually get to the point where their weapons are so powerful that they can blow up the entire island if they weren't careful; nations A and B convince nation C that nuclear weapons are off-limits and nation C naively complies, and gets conquered by nation B once they get rid of them all; superpower A develops remote-controlled machines which can fly anywhere and bomb anything, superpower B starts swallowing up the neighboring smaller nations, superpower A installs its own presidents in countries which don't agree with it. Eventually, everyone's a part of one of three tribes, it's illegal to think things you're not allowed to, you can be killed with a single order from the king, big brother is watching you, and 2 + 2 = 5.
So, either two things happen beyond that: the superpowers eat themselves alive and everyone dies from war, disease or famine, or the superpowers are discarded as a superstitious, illegitimate institution and people finally move out of the tribal island era and into an era of secular rationalism. Beyond that is anyone's guess.