First we explain why the Fathers of the First Ecumenical council are remembered on the Sunday preceding Pentecost. There is a very good reason. What follows is important for all Christians to know and live by - an exegesis of the beginning of the High priestly prayer of Jesus - John 17:1-13. Let us especially understand the promise: "Keep through thine own name those whom thou hast given me, that they may be one, as we are."