what are we supposed to say again when christians ask us where we get our morality?

[Tonus]

Care to elaborate?

As an infant, I probably picked some trinket or other item off of a table out of curiosity, only to be immediately and sharply admonished "not to take anything that isn't yours." This may have been reinforced by a slap on the hand. This is the earliest form of moral training, akin to god giving a commandment. For the infant and young child, the questions I mentioned earlier are at the heart of any consideration of theft. What did I stand to gain, and at what cost?

As I got older, I probably had the notion that theft is bad reinforced in any number of ways aside from the gain/cost calculation. People will discuss theft disdainfully, and few people ever speak of it in a positive manner (the few positive examples of theft would likely be in stories like Robin Hood, where the theft is driven by the desire to right another wrong, and not for selfish reasons). It's likely that I had things stolen from me as well, and the feelings involved reinforced the negative feelings regarding theft.

At some point along the way I was able to consider the reasons why I felt that theft is wrong and establish concepts like ownership and respect for the property of another and use these to develop a conscience that helps me to resist stealing even when the potential gain seems to outweigh the possible costs.

White Americans gained an awful lot through the antebellum slave trade of the 18th and 19th Centuries, the Germans gained a vast amount of scientific knowledge through conducting scientific tests on the Jewish people so were those acts morally justified then?

I think that the people who committed those acts justified them as moral and may have even believed that they were moral. I do not consider them moral acts, regardless of when they were committed, and I believe that most people today would feel the same.