There is really no room for debate, in my opinion, when it comes to this situation. He firmly believes you are misguided and must be brought back 'to the light,' and will keep doing so until he is discouraged in some way. Your Dad got through by not being worn down, so you will need to do the same. You might say to him, "I realize this is an important subject for you, but I am not interested in discussing it. I have thought carefully and independently about what beliefs suit me, and do not appreciate your insistence that I'm 'brainwashed' or just 'parroting' Dad's beliefs. It's frankly offensive, and I'm not going to listen to it anymore. Please do not try to talk to me about this again." And then tune him out. Leave the room/house, turn your back, begin talking to others, continue reading and ignore him, etc when he starts in on the topic. He will likely complain that you are being rude, at which point you can point out that he is also being rude by continuing to bring up subjects that you have told him point blank not to discuss with you.
The upside of this treatment is that you can do this in a public location as well as at home. If he's in your home when he starts, I would not think twice of attempting beandip/ignoring, and if he gets mad, politely escorting him to the door and sending him home. It's not embarrassing to me because I'm not the one with the problem, it's the other person. Just keep it up even though you are ready to strangle him because he's so inconsiderate of your choices. Eventually he will realize that it really IS that big of an issue for you and will make a choice accordingly.