It's Mister Slave, not Slade. Sorry, I just had to say that.
Chrissy can ignore whatever he wants. If he thinks South Park is installing some kind of normal, positive moral in his brain, he will automatically become an Olympic-grade mental gymnast and convince himself that the message being present is something that already fits his world view.
Also, why does Chris like Eric Cartman? That's easy. Cartman is a fat, bigoted, egotistical jerk who refuses to learn from others and will whatever it takes to impose his will on other people. Chris just sees so much of himself in Cartman that he can't help but like the character. And if Chris is a good Christian, then surely Eric is too.