It is something worthy of notice, amongst the remarkable things of the present time, that the Christian Sabbath, contrary to what would be the natural expectation in the case, is attempted to be set aside by persons who have a respect for religion, and appear to be persons of true benevolence and piety. Some of them make high claims to holiness of heart. The holiness of their hearts, as they understand it, has made all things holy. Their work is holy; their rest is holy; their recreations are holy, — everything they do, while the heart is holy, partakes of the character of the source or motive from which it proceeds. No one day, therefore, can be more holy to them than another. The Sabbath is on a footing with other days. All days are alike.
This is the general train of their thought and reasoning. And it cannot be doubted, I think, that there is not only a degree of plausibility, but a portion of real truth in these views.
It is true, in a certain sense undoubtedly, that all days, considered in reference to the subject of holiness, are alike. It would be absurd to suppose, that, while we are required to conform to holiness on one day, we are at liberty to deviate from it, in some degree, on another. It is true, therefore, that all days should be kept as holy as the Sabbath. And in this respect, and so far as this, all days are and ought to be alike. But it ought to be particularly remembered, while we admit that the requisition of holiness attaches itself to all days alike, and that one day is not and cannot be more holy than another; that they are alike by sameness of dispositions, and not by similarity of outward acts. They are alike to us, and are made alike in God's view, not by doing the same thing every day, but by doing that which is appropriate to the day. Time, in itself considered, is not holiness, nor can it be the subject of holiness. It is not possible that one day, in itself considered, should be more holy than another; but holiness consists in being and doing in time just that thing which is appropriate to the time. The law of God requires us to do everything with a holy heart every day, on other days of the week as well as on the Sabbath, and not more on the Sabbath than on other days. But this is a very different thing from doing or allowing the same thing to be done every day. The only true expression, therefore, the only true law, is, Do that which is appropriate to the time. Any known and deliberate violation of this law is sin; and cannot be otherwise than sin.
We are to do on the Sabbath day that which is appropriate to it. But it must be very obvious that the appropriateness of our acts can never be ascertained, independently of a regard to what takes place around us. The recurrence of the Sabbath, in consequence of what are understood to be the laws of God in the case, and of the general consent of all Christian nations, has the effect to stop the ordinary operations of life, and to hush the world to comparative peace; — so that there is a rest from physical labor, an opportunity to recover from undue exhaustion, and a season for moral and religious reflection and worship. It is a season, especially in the present condition of the human race, of immense, of incalculable importance. If, therefore, my recreation or my labor on the Sabbath day breaks in upon the general harmony, and disturbs the rest, the contemplations, and the worship of my neighbor, and thus does a serious injury to himself and his family, it is clearly inappropriate to the day. It is a violation of what is due from man to man, and is a sin.
— edited from A Treatise on Divine Union (1851) Part 7, Chapter 5.