You just made me laugh. Unfortunately, atheists have a better grasp of Christianity than Christians do. That's because a great many of us were one of the following.

1. Evangelical Christians (me)

2. Had a parent who was a pastor.

3. Did a lot of searching and reading.

4. Had atheist parents and learnt all the red flags to look for.

I think you're pretty much going to find it impossible to sneak up on us. While I'm aware that you want to witness the gospel, the thing is that people like me came to the realization that there is no god, and nothing you can say would ever convince us otherwise.