D. To encourage a consideration of ethics
Thomas Aquinas (1224 approx - 1274) is regarded as one of the Catholic Church's greatest theologians and philosophers, and he has even been honored as a saint in 1323 and named Doctor of the Church in 1567. Likewise, many studying for the priesthood regards him as a great model.
Therefore, Martin Luther's allusion to Aquinas is intended to encourage consideration of ethics or moral principles through the strong example of the words held by someone who had this same set of moral principles or ethics that promoted justice.