Can atheist and believers be best friends?
Sure. My late 2nd wife was a rural Methodist and I deconverted on her watch. It was of no big concern to her, and no threat. That's unusual, but then, she WAS special.
I think any relationship that's based on things besides church activities, theology, etc., can thrive independent of differences of opinion about religion. The world is MUCH bigger and more diverse than that. My late wife and I were both software developers (though she was no longer active due to disability) and had many interests in common, like hiking, travel, science ... I think the key is to find someone who, whatever existential beliefs they hold, do not confine themselves to being tribal around them.
OF course they can, I am an anti-theist my wife is a druid, I think she is stupid in that respect she thinks I am ignorant, but other than that we have a 27 year marriage, two kids (one is an atheist one is a born again) four grandkids and a plethora of mutual interests.
The problems only come when you make your religious/spiritual/ideological beliefs the centre and the whole of your life, which frankly is a waste of 99.9% of your life.
I grew up in a very religious community and still live in one. Never met an atheist before. So I didn't know exactly how y'all viewed believers to be honest. Thanks for your comment.
@Differentminded There's a pretty high percentage of people on a nationwide basis who belong to the "nones" -- the religiously unaffiliated. Currently over 25% and growing if I recall correctly. It's a big demographic trend. Nones can be atheists, agnostics, or non-practicing / non-joining theists. Some can be a bit nutty (some new age crystal gazers, for instance) but you definitely have a large group of people you may not be very aware of, who do not take religion very seriously and so can accommodate differences. You may have to find a way to reach outside your geographic region to fully experience it. Amarillo, Oklahoma City, and Albuquerque are within striking distance of you, so consider doing visits / activities in more urban settings too. Even in the Bible Belt / South, these are usually more inclusive and less uptight places.