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Not sure what to make of that other than George practiced under a Hindu Guru, so seems he thinks Christians are brainwashed.IMHO
I don’t think George thought that Christians were brainwashed, but I don’t know that much about his personal details.
George meant - Asking God to actually “wash our brains” - the opposite of “brainwashing” talked about in the other lyrics.
Harrison was a Hare Krishna
I don’t know much about it but I don’t think the lyrics are saying the God of Chirstians is brainwashing them.
Eastern religions teach stuff about emptying your mind so that I think is the idea George is trying to convey in simplest of terms.