A truth claim based on what?! Seriously.
In context of this part of the discussion, a truth claim based on whether Christianity is true or not. That would be an objective truth-- whether it is true or not. It will be either true or false. That's what you equated as an 'opinion' on my behalf.
Your 'opinion' is the opinion that there is no truth in the moral context. There is your truth, and his truth, and her truth, and Hitler's truth... And they are all equally valid. It is preference.
The difference between these 'opinions' is analogous to the opinion of whether OJ did it, and whether you like chocolate better than vanilla. OJ either did it or did not. There is no right/wrong answer to the best flavor of ice cream.
Now, we'll connect that back to you argument against my use of Christianity as an objective morality. Your claim is that, yes, your morality may be an opinion, but my belief in Christianity is just an opinion too, so we're all in the same boat.
But we are not. You are relying on complete relativity, and I am appealing to a truth claim. You are saying, in effect, 'everyone is right, who am I to say?' I am saying, "I believe I am right, and the objective, unchanging truth is the deciding factor.'
To further refine the context of application for this distinction... You and several others have subtly built the case that atheism is the 'safer' worldview than Christianity. But Christianity appeals to what it believes to be the moral truth. That moral truth includes the two laws: "Love God and love your neighbor as yourself-- neighbor defined especially as the person that you like the least."
With that moral law established, we can judge the actions of a professing Christian and see if they are consistent with the Christian moral law.
Atheism defines a relative moral law where no one is any more right than anyone else. The person's internal morality is correct, whatever that may be. There is no external, objective truth defined to judge self by, or to be judged from without. So, Hitler is free to do whatever he wants to Jews, because his view of Jews is all that matters, and who are you to tell him he is wrong? Stalin can do whatever he wants to detractors, because his truth says that human life is cheap for the cause of the party-- and who are you to judge him?