Fourth, the world will tell you that harmony means believing in everything and in nothing. Well, a popular idea around is that there is no absolute truth, and we must preserve harmony by never saying anything against someone else's beliefs. People will put pressure on you to believe that everything is true. Which of course is stupid, because some things contradict each other, so you end up believing nothing really. True harmony is not by watering down the truth.
True harmony can only come by sharing the truth together.
He bases what he says here on what he said about who we are in Christ. Christian love, unity, harmony, can only come from our common faith in Christ. By obeying the truth, as Peter puts it. Because of this idea, the world says you must never preach that only your way is right. Turn over a few pages to 1 John 5 verse 11.