So, are you saying these people are believers? I guess we might want to clarify that too, by believer do you mean a person that's been truly born-again of His Spirit? Someone really saved? If so, you're asking what if someone who was really saved could knowingly embrace a false doctrine? No, they couldn't. His Spirit indwelling them wouldn't let it ride; He would always guide the person to truth. I know His Spirit won't allow me to knowingly accept falsehoods and false doctrine I KNOW are false, but I as a believer have believed false doctrine in ignorance before He opens my eyes to its falsehood. So as far as "knowingly" accepting false doctrine, NO I don't believe we can, but can we in ignorance buy into and believe a false doctrine? Yes, at least I know I have, until He guided me to the TRUTH of the matter.