I don't see her as disrespectful at all. She reminded the person that the letter wasn't about biblical beliefs  (or that the LW didn't say so) but about being honest about her reasons for not wanting to attend the wedding. I loved her answer because it seemed like the person was trying to say that Amy was against the original LW having their own beleifs/convictions, which she never did and ecouraging someone to be honest is extremely biblical!!