Religion is simply a handicap for those who are afraid of not knowing what will happen to them after death, or at least it was for the people that created religion in the first place. Since people did not know what came after death, they created a mythical god or gods to make them feel secure. If humans lived forever, then religion would never have been created in the first place due to them never having a fear of death.
Religion encourages you to judge others who don't think like you, to not believe that the other ways are acceptable. Without the fear of punishment in the afterlife, there is no reason to obey a god. Hopefully some day people can focus on making the real world a nice place to be, instead of fighting over beliefs from stories.
If we live forever, the fear of hell and judgement would be gone. Then, the only purpose for religiosity would be to extract any principles and moral structures that could be useful to individuals and society. The elimination of God and religion would not create a society of debauchery as some fear. Without the fear of death or the wrath of God, humans could focus on living rather than dying.
People try to do good things for the paradise that they will receive after death. The bottom line is that people live their life so that they can come more to peace of death. So that they have somewhere to go after they die. If no one ever died then these people would not be living their life toward the afterlife the religion says that they will receive. For instance, going to heaven would be like a paradise for people. So they live their life so that they will be able to get the acceptance into heaven. Without this ending then religion would really have no focus point.
I actually don't know why people think of such different religion. I always believe in ONE GOD. But since people support religion knowing that life is of a short duration, it will be useless thinking that "Religion will be pointless if humans lived forever. That is why I am supporting against it.
That's it. Without the threat of Hell or any type of punishment from an all powerful god, then people would not feel the need to turn to religion. Many are already pushing religion aside since technology has advanced so much, and with their lives being more comfortable they won't need a god to feel comfortable.
Religion does not provide real answers backed up by evidence. Saying "God did it" is not sufficient enough for people to join willingly. If you allowed a person to live their entire lives without religion then told them when they became an adult, most likely they wouldn't believe it. That is why children become indoctrinated.
Without the constant reminder of eternal hellfire people wouldn't feel compelled to join a religion, so it would soon become a minority in the world.
Religion was most likely invented to explain things that we did not use to know, like the Sun, earthquakes, diseases, the beginning of the Universe and... Death. Admittedly, we still do not know what happens after we die (mostly likely nothing), but that is besides the point.
Some people might be tempted to say "well even if we lived forever, there would still be unknown things out there", which I would have to agree with, but here's the thing: death is the main reason why people cling to religion, not earthquakes, not the earth, DEATH.
One of the fundamental tenets of many religions is that we are all connected. This is actually the case: the world we live in depends on what we and our forebears have created. Religion teaches that we must act with selflessness, love and compassion to improve our lives and the lives of others.
Most religions work by means of expressing why things are and where they came from, two questions humans really have trouble answering and mentally handling, but what happens after we die is less related to death and more related to the unknown which the first two cover ("Why we die and what happens after?") rather than the concept of death itself.
Religion has waned if one takes note after the birth of Ethics.