The text of the Introit comes from Philippians 4: 4-5 and Psalm 84. In Philippians, St. Paul tells us: "Rejoice in the Lord always; again I say, rejoice. Let your forbearance be known to all men. The Lord is at hand. Do not be anxious over anything; but in all manner of prayer, let your requests be made known unto God."
Today's Gospel tells of John the Baptist, and his testimony that he is not the Christ. (This is today's Gospel for the Extraordinary Form of the Mass; it may have been different at the Ordinary Form of the Mass). John then explains to his inquirers who he is: "I am the voice of one crying out in the desert, make straight the way of the Lord, as Isaiah the prophet said."
Here is the image that accompanies this passage, which comes from the first chapter of St. John's Gospel, as it is found in the Sacred Art Series' The Holy Gospels of St. Luke and St. John.
On this Gaudete Sunday, let us heed the call of both St. Paul and St. John the Baptists: let us prepare ourselves for the coming of Christ at Christmas by removing the sin in our lives, and let us rejoice that the light of Christ will soon dawn at Christmas.