Listening to the exact same piece this year, the answer seemed so obvious to me. Let me see if I can say it as plainly as I felt it.
First, let me run those words past you again:
O thou that tellest good tidings to Zion, get thee up into the high mountain; O thou that tellest good tidings to Jerusalem, lift up thy voice with strength; lift it up, and be not afraid; say unto the cities of Judah, Behold your God! Arise, shine; for thy light is come, and the glory of the Lord is risen upon thee.The answer that has become obvious to me is that for me to fulfill this command to speak means for me to simply be myself.
Gloria Dei vivens homo: The glory of God is man fully alive. As far as I can tell, this quote first came from St. Irenaeus (from the 2nd century), but it was a favorite of Pope John Paul II, echoes in the writings of Vatican II, and is almost a summary essence of the heart of Fr. Giussani and the charism of CL. What does it mean? Glory is a very hard word to adequately define or fully paint, but it's not way off to say that the glory of God given on earth is the fulfillment of God's desire. When God is glorified, his purposes are being fulfilled.
When you take that view of glory and go back to these passages from Isaiah that Handel used, it becomes clear that they are Messianic texts. The light coming, the glory shining, the announcement to behold God -- they all point to a revelation of the Divine.
And after the fact, that is after the Messiah entered the world and the secret was out (that God's plan had been to reveal Himself through our very humanity by the Incarnation), Irenaeus was able to see it: This is what the glory of God is all about, man becoming fully alive. Jesus of course reveals the ultimate fully alive Man, and in Mary we see the seed of God's plan for the Church. This is the plan of the Trinity, accomplished through the Incarnation, the death and resurrection of Christ (and His ascension and the descent of the Holy Spirit). The plan is all about access to Divinity for every human person through the grace made available through the Church. The Church exists on earth so that every person may truly become fully alive, so that grace may work in our lives: soaking, healing, relieving, nourishing, reviving, as we together experience Him in our midst. Right here, right now, in this companionship, this family called the Church, this is where we meet Him and this is where the process begins. (And fortunately we're not doomed to have to complete all the soaking here on earth or be stuck forever! That's the glory of purgatory.)
So. What does God ask of me? What does He desire for my life? He wants me to be truly and fully alive. Giussani would say He wants me to be completely free -- to seek after the destiny, the precise purpose for which He made my life is making my life. That destiny is God Himself, ultimately. But how do I know the path to follow in this life, my vocation, my calling? In the most simple sense, the calling of each person is to be the person God has created them to be. God strongly desires us to be individuals! Not individualists, to be sure -- not Lone Rangers or self-made persons. I am neither a Rock nor an Island. But I am me, and this is what God wants. Being me is the service I am to render to God and to the world.
I came across a quote recently from Msgr. Lorenzo Albacete which goes like this: "When you are no longer afraid to be yourself in front of other people, then you are really free. Otherwise, others determine you." This resonates with me strongly, and to be it echoes Isaiah's words: "lift up thy voice with strength; lift it up, and be not afraid; say unto the cities of Judah, Behold your God!" When I live my life as under the judgment of Christ alone, not even subjecting myself to my own interior censors, but only to the objectivity of the gospel, that is when I am no longer afraid to be myself. That's when I don't change my behavior because I'm with a different group of people. That's when I am free. That's when the glory of God is revealed in me. That's when, without saying a word, or rather by saying whatever words the normal functioning of my life requires of me, I proclaim "Behold your God!"
I'm glad, though, that the grace of God can and does work through imperfect earthen vessels. God would really be up a creek frankly if grace only could work through those who had already been wholly perfected! To grasp this is to understand the teaching of purgatory as pure mercy.
What seems so marvelous to me in this realization is that God isn't asking us to buff up on some method, some program of communication, some evangelization curriculum that is exterior to us, like a big clunky overcoat. Essentially we need to repent, believe and follow when we meet Him in our midst. We will change if we just stick with Him. It is the change in us that results from being with Him that we need, and that the world needs to see in us.
P.S. Oddly, this post has become one receiving the most hits of any I've written. I am edified by that, and just want to add that I wrote this the day after miscarrying my last child. Also, I had no idea at the time that this had almost a prophetic importance to me, as just a few weeks after writing this, I entered into an entirely new spiritual odyssey through which I've discovered how much more "fully alive" I could be. To read about it, just keep reading forward from this date.