the nature of sin

[Drich]

The christianese "written on our hearts" is conceptually equivalent to the secular statement that human beings possess moral agency.  

With the exception of those who genuinely believe in the doctrine of total depravity, there is no fundamental disagreement between the christian view of morality or moral agency and the secular view.  Unsurprising, again, due to the syncretic nature of christian belief.  

They heard what the pagan philosophers of classical antiquity had to say about moral agency and thought it was legit - they disagreed about gods.  We (the unfaithful) also dip into that same pool when we make moral assessments or proclamations.

are you kidding? when these 
'laws' were put into place they were not well received as it gave women rights secular law did not give them it gave slaves laws and right they never had, it restricted men's sexual activity making much of it forbidden and it put limitations of people and expectations no one else had to endure. in a culture where gods could be bribed and forgiveness be bought  by paying off priests, these laws demanded accountability in this life and in the next.

Then Christ came along and extended them to include thought to force 'good people' to admit they were not indeed good. what secular law did that? rather secular law seeks to make moral our wants and perversions so long as they do not hurt anyone in the short term.