When debating for the Atheist point of view, a common problem encountered is that many counter arguments to "religious experiences" completely nullify the other party's experiences. Arguments which do this are not very persuasive, and convince the other party that you don't understand their point of view. Here's how to be more convincing with your arguments:
Allow me to first clarify "religious experience" or a "supernatural experience". Here is just one highly under-embellished example: "I was hit by a car when I was younger, and while I was unconscious, my vision faded from black to white, and I heard god telling me it was not my time to die and I needed to wake up, and now I'm alive".
Fucking bullshit right?
The problem is, with whoever is the author of this story, they know this happened. This story might seem crazy to the logical thinker, but to the person who actually experienced it, it is as real as any other experience they have had. And thus, just plainly denying their experience as impossible, is not convincing, and shows that you must not understand.
Imagine someone saying the breakfast you ate this morning wasn't real. Everything you tasted, the temperature, they were just illusions in your head. But you know you ate breakfast, it was delicious (or not, whatever). This is what it's like plainly denying their experience by saying "god isn't real, so that's bullshit".
A better approach to this situation is to validate their experience, and offer alternate explanations. "That sounds amazing, but how do you know it was god? And not just a dream? If you were dreaming how would you know it wasn't a dream?".
Generally, if someone has attributed an experience to something supernatural, they obviously do not know of anything natural that could have caused the experience. This could be due to a lack of knowledge by the person, or by science/ society in general.
The brain can do some amazing things, dreaming, hallucinations, all which are very real for the mind creating them. Remember to take this into account before calling bullshit on someone's experience.