John was the forerunner for Jesus. His role, his one job, was to prepare the way for the Messiah and to announce his arrival. He told people that the Messiah stood among them and they did now know him.
Knowing Jesus is the point of discipleship. Knowing another person takes familiarity and study. Familiarity comes from spending time together and study leads to deeper appreciation. A popular book for marriage these past few years is "The FIve Love Languages". Truly there is a lot of effort that goes into loving and appreciating others. Knowing Jesus and being known by him is not a mystery or particulary difficult thing to do, and it is our Advent discipline.