Isaiah 66:10-14; Psalm 66:1-3, 4-5, 6-7, 16, 20; Galatians 6:14-18; Luke 10:1-12, 17-20
There is a theme of rejoicing in the readings for this Sunday, specifically in the first reading and the psalm. Indeed, the first reading includes the beautiful text in Isaiah from which Laetare Sunday in Lent drew its name: "Rejoice with Jerusalem and be glad because of her, all you who love her; exult, exult with her, all you who were mourning over her!"
The entire first reading is filled with a message of joy and comfort from God to His people. The psalm continues this theme in a context of giving thanksgiving, and above all praise, to God. Throughout the reading and the psalm there is a consciousness that the great deeds God has done for His people ultimately serve the glory of God. This is always something for us to keep in mind, of course: God's gifts are not meant to terminate in my isolated self.
The theme of rejoicing is also picked up in a way in the Gospel, although you may not notice it if you hear only the shorter version of this Gospel at Mass. The shorter version omits the end contained in the longer version, in which the seventy-two disciples whom Jesus sent out at the beginning return to Jesus rejoicing. Jesus affirms that there is great cause for joy, while gently reminding them that they should rejoice primarily not over the power given to them, but at the greater and more fundamental gift they have received: "Nevertheless, do not rejoice because the spirits are subject to you, but rejoice because your names are written in heaven." It is the gift of a place in the kingdom of God that is the most fundamental cause for joy.
It is interesting in this regard that Jesus actually tells the seventy-two disciples that in the end they are to give the same message to both the towns that accept them and the towns that reject them: the kingdom of God is at hand. However, for those that accept them this is a message of hope, accompanied by healings, while in the towns that reject the disciples it is a message of condemnation, and a warning that the Kingdom is at hand, regardless of their rejection. The Kingdom of God does not depend on my acceptance of it, but a place in the Kingdom will not be forced on me either.
Furthermore, note that we do not just accept Christ in an individual relationship with Him. The mediation of the mission of Christ through the disciples here illustrates that accepting Christ, and accepting the gift of the Kingdom, also means accepting the Church as mediating Christ to us, in the teaching of the Church, her sacramental life, and so forth. If I reject this role of the Church, with all the apparent flaws and imperfections involved, I risk losing the ultimate basis for all the joy and comfort shown in the beautiful first reading from Isaiah.
(These comments were written for the same readings in a previous year, and are being re-posted now.)

