God: "I looked for someone to take a stand for me, and stand in the gap" (Ezekiel 22:30)
Your conscience is like the nerve endings in your fingertips. Its sensitivity to external stimuli can be damaged by the buildup of calluses or even wounded so badly as to be virtually impervious to any feeling. Paul also wrote of the dangers of a calloused conscience (1 Corinthians 8:10), a wounded conscience (v. 12), and a seared conscience (1 Timothy 4:2).
In John 13 the point was that, if an individual Christian does not show love toward other true Christians, the world has a right to judge that he is not a Christian. Here (in John 17:21) Jesus is stating something else which is much more cutting, much more profound: We cannot expect the world to believe the Father sent the Son, that Jesus' claims are true, and that Christianity is true, unless the world sees some reality of the oneness of true Christians.
