There is nothing immutable except what has come from God. Everything that is the work of humans is subject to change. The laws of nature are the same at all times and in all countries. Human  laws change according to time, place and intellectual progress. In marriage, what is of divine order is the union of the sexes to carry out the replacement of people who die. However, the conditions that govern this union are of an order that is so human  that  throughout  the  entire world –  even throughout Christendom – there are no two countries where such conditions are completely the same, nor is there one where they have not undergone change over time. The result is that, in the eyes of civil law, what is legal in one country and at a given time is adulterous in another country and at a different time. This is because civil law is meant to regulate family interests, and these interests vary according to customs and local needs. Thus it is, for example, that in certain countries, only religious marriage is legal; in others, a civil marriage is also required; in others, finally, a civil marriage is enough by itself.