You decide. We all decide.

On an individual basis, you can decide if you think an action is ethical or not based on if it, for instance, causes harm, and you dislike causing harm to others.

As a society, we broadly come to a consensus on what we consider ethical or not by majority opinion, and turn those into laws. It’s why murder is considered wrong, in both religious and non-religious institutions and societies at large.

For example, as a society, we deemed killing other humans to be wrong because then we would be at risk of being killed, and it made it harder for us to survive overall. Those who killed were ostracized, those who didn’t were not. No religion was required to form such a belief, but it can certainly be a part of religious teachings.

You can use the Bible as a framework for how you decide what’s moral or not, but it’s not the only way to do so.