You'd better hunker down for the long haul, because I deconverted 20 years ago and my immediate family is still convinced it's a "phase" and/or a "misunderstanding".

In my experience people who knew me well when I was an exemplary and devoted believer, explain it to themselves that way. Those who didn't know me when, assume I'm making up a story about myself having been a former believer, or a believer of any credibility at least, and deploy the circular reasoning supported directly by scripture: I was never really one of them to begin with.

ANYTHING but to admit that atheism could be a considered position. It has to be a "misunderstanding" or a lie or rebellion or hatred or deviousness of some kind.

If you have any need to be understood or respected, now would be a good time to let go of that

Don't worry about them. Who cares what they believe?

Whenever someone brings up religion, just shrug and remark that's not how you believe now. Then immediately change the subject before they can argue. If they try to follow you to argue, just act distracted and vague, murmur "um-hum" vaguely as though humoring a young child, then wander out of the room.
Only engage with them when they don't talk about religion. It should soon train them to leave the subject alone.