My point in raising the passage from Mark 10 is that Jesus associates the nature of good with God exclusively. Therefore, the idea that God is good goes back to the beginning of Christianity itself.

And so, when a Christian today defends God's actions by saying "Whatever God does is good because God is good," that Christian may be clumsily arguing the point. He or she may not even fully understand the point they are making. But...

It is a point (however awkward or incomplete it's made today by various people) that goes straight back to Jesus.