Agreed. I find that people who argue that religion is necessary for ethics tend to ignore the history of their religion and the fact that the original text largely serves as a jumping off point for religious philosophers to connect older “secular” texts to this new religion. Modern Christianity is a complex combination of Platonic, Aristotelian, Syrian, and Roman ideals which are taken out of their original context to align with the Bible even though the original writers would say they knew nothing about Jesus. The base texts which many of these ideas are based on make almost no appeals to God and focus more on what it means to live a “good life”. To be fair a lot of great ethical arguments are made by Christian writers but I think that’s more just a consequence of their cultural upbringing and the fact that the thing the New Testament really added to the discussion was that your ethical responsibilities generalize beyond yourself and your friends/family.

Religious ethics are just as fluid and complex as secular ethics, it’s just that the concept of God makes people think they can claim that their way of thinking is the only one that’s real. I would guess if you self-reflect though you’d see that even within one lifetime the definition of what’s moral in a religious context changes as well.