"What God has joined together, man must not separate” (Matthew 19:5).
We see in Jesus' teaching about marriage his knowledge of the scriptures serving as continuity and precedent. He quotes Genesis about the ideal union of man and woman as the full image of God imprinted on humanity before sin enters the picture, insinuating the division that has confounded relations among the sexes ever since.
But Jesus also knew the great narrative of the Prophet Ezekiel about how nuptial love represents God's covenant with Israel. It is because this faithful union is so important to salvation history that those marriages that God has blessed with the divine image must be signs of indissolubility. As God never stops loving Israel, so husband and wife must reflect that kind of fidelity.
Yet even Ezekiel includes the frailty of human love that often fails. Israel was in fact unfaithful, practicing idolatry and forming alliances with pagan neighbors instead of trusting exclusively on God. So the narrative includes God's steadfast love even in the course of human failure. The ultimate revelation of God is less about our perfection than about mercy. God forgives and heals when we sin and wound our relationships. The community of the Covenant witnesses to this ongoing history of mercy.
In the debates over the indissolubility of sacramental marriages at the synods on the family, one focus was on the power of mercy to encompass human failure and hold people within the covenant even when they fall short of the ideal. The question, even for Canon Law, is just how many unions have been "joined by God," among the many attempts by people to constitute a true bond, freely entered into with full knowledge and maturity. It may be that only marriages that last 25 years can be held up as signs of the Covenant, especially if that is why they strive to witness indissolubility to the rest of the community.
We all walk in the mercy of God, who alone knows the hidden dimensions of our intent and effort to be faithful to the graces we have received, especially in our relationships. We know this for certain, that God is always faithful and never stops loving us, no matter what.