Ah, I see.

It seemed like you were feeling bad for not speaking up to support Christianity in that moment... but as an atheist I suspect doing so would have only reinforced her point.

In my opinion it's OK sometimes to let people say things that conflict with your values. You probably won't change their mind by disagreeing, and maybe it's more important to build trust with your new friend by letting them feel heard.

Maybe your wish to avoid conflict with her was not putting selfish desires before God or denying God, but rather a healthy intuition that it would have done more harm than good in your relationship.

But if it revealed to you that maybe you weren't sure how you felt about Christianity, then I see how that could be alarming.