dts:
I am seeking resources, especially online resources, that discuss the historical/theological development of marriage as a sacrament? When was it first explicitly discussed as a sacrament outside of Scripture?
Have you checked the Catechism of the Catholic Church? Check under Marriage and Matrimony. Also, the Vatican website has tons of materials, including the Catechism. Check it out at
www.vatican.va or the with the National Conference of Bishops at
www.nccbuscc.org I would also look up Marriage/Matrimony under
www.newadvent.org
No. 1601 of the Catechism states, "The matrimonial covenant, by which a man and a woman establish between themselves a partnership of the whole of life, is by its nature ordered toward the good of the spouses and the procreation and education of offspring; this covenant between baptized persons has been raised by Christ the Lord to the dignity of a sacrament.
Footnote 84 associated with this references CanonLaw #1055 & the document Gaudium et spes 48, paragraph 1, all of which are available through the Vatican.
Matrimony was not raised to a sacrament by Church officially until the Second Council of Lyons, although it had a hand in it prior. The Council of Trent (1545-1563) solidified it. So do a search on these Councils also. After Vatican II with the 1983 revised Code of Canon law, more was added to safeguard the sacredness.