Why do Christians associate the New Testament Christianity with ethics and morality
Contra-the other perspectives in the thread, the Christian connection to ethics is not ungrounded. Quite to the contrary there are at least four reasons why Christians might make this supposition.
History. Historically the New Testament moved from a retributive ethics (or even more revenge ethics) to one grounded in empathy, dignity, and forgiveness.
Cultural. Kantian ethics was grounded in or at least heavily influenced by Biblical morality. Kantian morality went on to be the basis of how we view ethics in everyday life and in government (honesty, respect, and fairness).
Their experiential proof. They’ve sinned and they’ved walked in the model of Jesus. They’ve found the later more truthful and resonant with their very being.
They see the value and self-evidence truth of virtue in building better humans, family, and communities. Rome fell, because of its excess, idolatry and lack of understanding of these principles.
They find the alternatives to be suspect. A purely secular ethics tends to be grounded in pleasure alone on the one hand or radical individualism not subject to consequences–Christians feel this ethical orientation–most evident in Gadspy esque and 1% living. Radical autonomy and anything goes–is hardly an ethical system–much less a prescription for healthy living and communities.