How far our culture is from that of the church when we talk on marriage. In a recent group of secondary school youth people, the idea that you can only marry once seemed shocking. It was an idea that was unrelatable to them and their experience. We can't presume that no divorce is held as even an ideal, so our catechesis needs to address the objectors, while constantly being grounded in the core Gospel message.
Marriage is a sacrament of love. It is an outward sign to make visible the love of Jesus for us, his Church. That faithful and complete self-giving love. At the recent first Holy Communion Mass in the parish, the girls all wore white dresses with veils. See, there is an understanding that the Marriage makes sense of the Eucharist. In the Eucharist, Jesus gives himself to us. In Marriage, we give ourselves to our spouse. That's why the minister of the sacrament of marriage is the spouses (the priest or deacon serves as a witness and leads the Liturgy).
Marriage is about union. Holy Communion - this involves being in communion with God and his Church - that's why the Church has 'closed communion' and Catholic's don't receive in an non-catholic eucharist.
The most useful images of the Trinity come in married life. The interpenetration of persons (whereby the Father contains the Son and Spirit, but also the Spirit contains both the Father and Son, etc) is perhaps relatable to pregnancy (where the child is contained in the mother and the child contains the mother's dna). Secondly, we see the Trinity. Just as the love of husband and wife is able to generate a new person, the love of God the Father and God the Son is so powerful that it exists in its own right as a person (whom we know as the Holy Spirit).
The Church's vision for marriage is so much higher than society's. It demands giving rather than taking. It is a faithful covenant, rather than a legal contract. A contract is an exchange of goods/services, but human persons are not goods to be traded. A contract is broken, if the other party fails to meet his terms (for instance, if I didn't pay my phone bill, the network won't honour their commitment to provide the service). A covenant is a complete self-gift to another. God is ever faithful to his side of a covenant, even when the chosen people fall short. This is why marriage is such a powerful image of God. Hosea married a prostitute, which Scripture used to illustrate the infidelity of Israel through the infidelity of Hosea's wife.
We need to teach this. It is good news. It's also a way to speak God into our culture which is very sexualised. And if we fail, the Church's teaching becomes a stumbling block to encountering the love of the Father. We talk of the scandal of the cross, and perhaps the church's relentless pursuit of purity will be a scandal in that sense - a sign of contradiction as Humanae Vitae commented - the synod might shock people expecting a change in doctrine. But God has not abandoned his Church. Take heart, don't be swayed and bothered by the trends of the day. Don't be knocked off course. Hold firm to the Church. Be docile to the Spirit - but discern and be humble always.