I don’t understand the first half of your post.
People grow up in different countries with different religions. We can’t, for example, expect everyone living in Russia in 1940 to have much exposure to, say, Hinduism. There is usually a “majority” religion where a person grows up and they “assent” to it because it is around them and their parents follow it and they are taught it.
In my first half, I was addressing the issues of reasonable assent. One can only assent to a religion, even one passed down through parents if it is reasonable. At a very young age, it would be reasonable to assent to parents regarding the religion they proclaim as true. Here, the child accepts parental authority (i.e. that parents know what is best and they have done their homework on why they assented in the first place) But once mature, one must find the actual reason to assent to a specific religion(or ask why your parents assented to it).
So while there are countless religions (including those you and I can start tomorrow), none of them can give reasons to consider their view to be reflective of the transcendent or that their founder has any clue regarding the transcendent. Hence they are all arbitrary (albeit logically consistent frameworks) and not worthy of assent.
In this sense, even Atheism is just as bad. Here you have followers who give assent to the claim that “There is no Transcendent/God(s)” when the claim itself is not within the directly verifiable scope of knowledge. Hence it is unreasonable. The Atheist has much chance of knowing about the non-existence of the Transcendent as a believer has of describing the transcendent by his own effort (i.e. without faith). So to give assent to the claim of atheism is as bad giving assent to the description of the transcendent by some arbitrary individual or book.
Re the second half…you bring up murder as an example of something that is either right or wrong in religion, but could be changeable in a secular society.
But murder is a good example of moral relativism in religion.
There are many parts of the bible that condone and encourage killing and genocide…and at the same time, it urges: “thou shalt not kill”.
In some religious teachings, birth control/abortion is a sin…but you must be willing to murder your child if God commands it.
So, using your example of murder…its level of morality is indeed very, very changeable in the bible "depending on the times and needs, " as you say.
And the bible is not secular.
I am not sure what you are arguing against here. Murder is acting contrary to God’s will to end another’s life. If it is the will of God that a whole race be abolished from the earth, and he would like some persons to carry it out, it would be moral for them to do so.
So the issue here is since God’s will / Morality is a transcendent quantity even if it did exist, we cannot say for sure what is moral or immoral. We can perhaps draw consensus on things like murder ( here i mean **needless **killing of innocents ) but that does not give us reason to think that our consensus reflects the transcendent reality or if it even exists.
A word should probably be mentioned here about murder in the above use of the term. You might think at first that there is actually no consensus with respect to murder since there are so many cultures that practiced child/adult human sacrifice, cannibalism etc. But this would be to miss the point. All human civilizations accept that killing needlessly is wrong. Even those who killed their children as sacrifice only did so because they felt it necessary for their God’s.
Even in the case of pro-abortion individuals today, they argue on the basis that the fetus is not human. Or they (probably very few) would argue that the fact that the human is located in the womb inside the mother, give the mother the right to end the human life.
But in all cases, there is giving of reason for ending of life. No one accepts that it is ok to randomly kill for no rhyme or reason another human being. There is a consensus on that.
That being said, as I said before, this is just consensus based on intuition. Unless someone can be given reason to believe that this consensus based on intuition means anything, its purely useless in terms of knowing what actual morality is.
My argument is that most religions fail on this aspect. They cannot give any reasons to give assent to their holy text or founder by giving reasons to think them/it authoritative regarding the transcendent. Hence, no religion other than Judaism or Christianity (specifically Catholicism (in communion with Rome) and to some degree other Christian faiths) is reasonable to assent to.