This particular false assumption is quite insidious. The best line of argumentation is to point out that it is reasonable to reject a person's point of view without rejecting the person. Bigotry is the rejection of the person along with their point of view. For example, rejecting, or denouncing a person who believes that Jesus is the only way to heaven, is bigotry. Sincerity is all that matters As long as you're sincere in what you believe you will have God's acceptance.
Sincerity is terribly important.
I have met many church people who are obviously not sincere in their faith. Church for them was a source of social stimulation. The trouble is you can be sincerely wrong. What a tremendous loss it would be to put your faith in a religion based on lies and so lose the very thing you search after. The way that leads to destruction is broad and crowded.