He's cast the gospel in terms of psychology and philosophy rather than terms of morality. Sin is a moral rebellion against a moral God, and to neglect or diminish that element of sin is not to preach the true gospel of the New Testament. The existential gospel says man is finite and insecure and needs cosmic help. That's quite different than man is sinful and rebellious and needs moral deliverence. Paul Vitz points out the peculiarly American strains in Rev. Schuller's ideas.
American existentialism is optimistic, European is pessimistic, but the assumptions are the same.
Obviously, if one of the important ways you're supposed to be made is to be saved, you're the self-made man. It appeals to our vanity, or basically, our narcissism. There is common agreement among all those interviewed that salvation is the bottom line. Naturally people love it because he doesn't preach about sin, judgment, or anything negative, just about your own possibilities. But to avoid the offense of the gospel you have to avoid the gospel.