Human dignity isn’t derived from secular society, but from Judeo-Christian scriptures. According to the Bible, there are no mere mortals. All humans inherit the image and likeness of God, which should motivate us to treat everyone with respect.
This perspective of human dignity is the foundation of such institutions as individual liberty, private property, contracts, law, universities, hospitals, and charity. We serve people in honor of the God who created them and loves them regardless of their spiritual condition and eternal destiny.