What church someone chooses to attend is none of your business unless ‘they choose’ to say so...or for that matter if they do or do not align with any denomination...there are many churches that are not mainstream....and many that have broken away from them after "World Council of Churches" began their push.
Aryan nations, Jehovah’s witness, The Way, Branch Davidian, 7th Day Adventist, Mormonish, etc., etc.