From Charles Hodge's Systematic Theology, Vol. 3
The proposition that the United States of America are a Christian and Protestant nation, is not so much the assertion of a principle as the statement of a fact. That fact is not simply that the great majority of the people are Christians and Protestants, but that the organic life, the institutions, laws, and official action of the government, whether that action be legislative, judicial, or executive, is, and of right should be, and in fact must be, in accordance with the principles of Protestant Christianity.
1.This is a Christian and Protestant nation in the sense stated in virtue of a universal and necessary law. If you plant an acorn, you get an oak. If you plant a cedar, you get a cedar. If a country be settled by Pagans or Mohammedans, it develops into a Pagan or Mohammedan (Muslim) community. By the same law, if a country be taken possession of and settled by Protestant Christians, the nation which they come to constitute must be Protestant and Christian. This country was settled by Protestants. For the first hundred years of our history they constituted almost the only element of our population. As a matter of course, they were governed by their religion as individuals, in their families, and in all the associations for business, and for municipal, state and national government. This was just as much a matter of necessity as that they should act morally in all these different relations.
2. It is a historical fact that Protestant Christianity is the law of the land, and has been from the beginning. As the great majority of the early settlers of the country were from Great Britain, they declared that the common law of England should be the law here. But Christianity is the basis of the common law of England, and is therefore of the law of this country; and so our courts have repeatedly decided. It is so not merely because of such decisions. Courts cannot reverse facts. Protestant Christianity has been, is, and must be the law of the land. Whatever Protestant Christianity forbids, the law of the land (within its sphere, i.e., within the sphere in which civil authority may appropriately act) forbids.
Christianity forbids polygamy and arbitrary divorce, so does the civil law. Romanism forbids divorce even on the ground of adultery; Protestantism admits it on that ground. The laws of all the states conform in this matter to the Protestant rule. Christianity forbids all unnecessary labour, or the transaction of worldly business, on the Lord’s Day; that day accordingly is a dies non, throughout the land. No contract is binding, made on that day. No debt can be collected on the Christian Sabbath. If a man hires himself for any service by the month or year, he cannot be required to labor on that day. All public offices are closed, and all official business is suspended. From Maine to Georgia, from ocean to ocean, one day in the week, by the law of God and by the law of the land, the people rest.
...The demands of those who require that religion, and especially Christianity, should be ignored in our national, state and municipal laws, are not only unreasonable but they are in the highest degree unjust and tyrannical.