In Luke 9:27-36, three of the Lord's disciples witnessed His transfiguration on Mount Hermon. Before and after this great event, Jesus revealed to His disciples that He must go up to Jerusalem to be delivered up and crucified, so that He could perform His redemptive work in God's economy.
Yet none of this had any real impact on His disciples. Instead, they were too busy contending among themselves as to who was the greatest amongst them. The Lord needed to teach them concerning humility - that is, not thinking overly much of yourself - and tolerance - being able to accept people who don't follow everything you do, for the sake of keeping the oneness:
And He said to them, Whoever receives this little child because of My name receives Me; and whoever receives Me receives Him who sent Me; for he who is least among you all, this one is great. And John answered and said, Master, we saw someone casting out demons in Your name, and we forbade him, because he does not follow with us. But Jesus said to him, Do not forbid him, for he who is not against you is for you.
One of the biggest problems for Christians is the matter of humility and tolerance, especially of other Christians. To many divisions exist because believers insist that they are right, that their way is right, and that if you don't follow their way, then you are not a true believer. According to the Lord's teaching, we should be like little children, who are unburdened with such high concepts, and instead are able to take on a new thought.