Are you telling me that you believe that "the cross" exempts you from all of the things that Jesus taught even during his life?
As a gentile believer, yes, i am exempt of some of the things contained in the law for the jews.
Why do you suppose he spent so much time teaching if that was the plan. You'd think maybe he'd throw in a caveat about that, or something. Like "forget all this compassion in three years, it won't apply anymore."
Because He was fulfilling several OT prophecies and proving to them He was messiah. Maybe this verse will help on this issue:
Matthew 15:24 But He answered and said, "I was sent only to the lost sheep of the house of Israel."
You do not understand what you speak about.
As for why an atheist might appeal to the authority of the Bible of Jesus, the reason is simple. It's not a moral authority, to me, although it is in this case morally persuasive. But it is an authority on Christianity.
In other words, you see it and know its the truth, like God said you would.
Romans 1:19 since what may be known about God is plain to them, because God has made it plain to them.
Romans 2:14 Indeed, when Gentiles, who do not have the Law, do by nature what the Law requires, they are a law to themselves, even though they do not have the Law, 15since they show that the work of the Law is written on their hearts, their consciences also bearing witness, and their thoughts either accusing or defending them.
You appeal to authority in the bible because you KNOW God is the authority, even when you misapply what is written.