When I was going through the forest, I happened to see a herd of deer.

I have to identify which part of this sentence has an error. In the book where I found it says that using ‘happen to’ is the error here, but doesn’t explain enough. Why am I confused? I don’t understand how to use a conjunction like ‘has happened to’. But I do know that that is still the case. Can someone not understand me?

How many people that did a religion consider themselves Christians?