I read today Augustine sermon (Sermon 216; PL 38, 1076-1082) for the competentes, that is, the people who had enrolled their names for baptism during the Easter Vigil. Some things that struck me:Early on, he calls them contirones mei, my fellow recruits, a nice touch, found elsewhere when he says that he and the members of his congregation are classmates in the same school whose teacher is Christ. A first metaphor for them to consider:

At the auction, the market place, of faith the kingdom of heaven is offered on sale to you. Take a look at it, and put together the resources of your conscience, gather the treasures of your hearts. And yet, you are buying without cost if you recognize the free grace that is being offered to you. You spend nothing, and yet you acquire something great. Dont hold yourselves cheap if the Creator of all things, your Creator, considers you so dear that everyday he pours out the most precious blood of his own Son for you. You will not be cheap if you distinguish the precious from the cheap, if you serve the Creator, not the creature.

At a later point, he contrasts the two cities:

Take on the world, be reshaped for God. May your Babylonian captivity now disgust you. Look: there is Jerusalem, that heavenly city, cheerfully coming to meet you on the way, and begging you to choose life and to desire to see good days, days like none you have ever had nor ever will have in this world.

The maternal image is picked up still later, but with different resonance:

Hope in God, the entire group of the new people, the people who are being born, the people whom the Lord has made. Strive to be born in healthy condition lest you be aborted and die. See the womb of Mother Church; see how she groans in her labor so that she may give birth to you, bring you out into the light of faith. Dont by your impatience disturb her maternal womb and narrow the gates through which you must pass. O people, who are being created, praise your God; praise, you who are being created, praise your Lord.... As it is written: Make your father glad by your progress in wisdom, and dont by your failure sadden your mother (see Prov 10:1; 15-20).

Here, as elsewhere, Mother Church is the Church that will give birth at the baptism of the new believers. They are nearing their birth-day, and she is already groaning as her labor draws near. St. Augustine meant all this very concretely. As he said in another sermon: Individually, we are all children of Mother Church; taken together, we are Mother Church. Do we think of ourselves, together, as about to give birth at the Easter Vigil?There is a very interesting article on the last stages of preparation for baptism in the time of St. Augustine: http://people.vanderbilt.edu/~james.p.burns/chroma/baptism/merdbapt.html