The separation into moral and ceremonial law is a common Christian view not just among Adventists, as near as I can tell it was developed by Thomas Aquinas though I have never took the time to document that for certain. It is old however.
Yes, as far as I can tell, it did come from Thomas Aquinas, who wrote the following in
Summa Theologica (articles 3 and 4):
On the contrary, It is written (Dt. 4:13, 14): "Ten words . . . He wrote in two tables of stone; and He commanded me at that time that I should teach you the ceremonies and judgments which you shall do." But the ten commandments of the Law are moral precepts. Therefore besides the moral precepts there are others which are ceremonial.
I answer that, As stated above (A[2]), the Divine law is instituted chiefly in order to direct men to God; while human law is instituted chiefly in order to direct men in relation to one another. Hence human laws have not concerned themselves with the institution of anything relating to Divine worship except as affecting the common good of mankind: and for this reason they have devised many institutions relating to Divine matters, according as it seemed expedient for the formation of human morals; as may be seen in the rites of the Gentiles. On the other hand the Divine law directed men to one another according to the demands of that order whereby man is directed to God, which order was the chief aim of that law. Now man is directed to God not only by the interior acts of the mind, which are faith, hope, and love, but also by certain external works, whereby man makes profession of his subjection to God: and it is these works that are said to belong to the Divine worship. This worship is called "ceremony" [the munia, i.e. gifts] of Ceres (who was the goddess of fruits), as some say: because, at first, offerings were made to God from the fruits: or because, as Valerius Maximus states [*Fact. et Dict. Memor. i, 1], the word "ceremony" was introduced among the Latins, to signify the Divine worship, being derived from a town near Rome called "Caere": since, when Rome was taken by the Gauls, the sacred chattels of the Romans were taken thither and most carefully preserved. Accordingly those precepts of the Law which refer to the Divine worship are specially called ceremonial.
Reply to Objection 1: Human acts extend also to the Divine worship: and therefore the Old Law given to man contains precepts about these matters also.
Reply to Objection 2: As stated above (Q[91], A[3]), the precepts of the natural law are general, and require to be determined: and they are determined both by human law and by Divine law. And just as these very determinations which are made by human law are said to be, not of natural, but of positive law; so the determinations of the precepts of the natural law, effected by the Divine law, are distinct from the moral precepts which belong to the natural law. Wherefore to worship God, since it is an act of virtue, belongs to a moral precept; but the determination of this precept, namely that He is to be worshipped by such and such sacrifices, and such and such offerings, belongs to the ceremonial precepts. Consequently the ceremonial precepts are distinct from the moral precepts. . . .
On the contrary, It is written (Dt. 6:1): "These are the precepts and ceremonies, and judgments": where "precepts" stands for "moral precepts" antonomastically. Therefore there are judicial precepts besides moral and ceremonial precepts.
I answer that, As stated above (AA[2],3), it belongs to the Divine law to direct men to one another and to God. Now each of these belongs in the abstract to the dictates of the natural law, to which dictates the moral precepts are to be referred: yet each of them has to be determined by Divine or human law, because naturally known principles are universal, both in speculative and in practical matters. Accordingly just as the determination of the universal principle about Divine worship is effected by the ceremonial precepts, so the determination of the general precepts of that justice which is to be observed among men is effected by the judicial precepts.
We must therefore distinguish three kinds of precept in the Old Law; viz. "moral" precepts, which are dictated by the natural law; "ceremonial" precepts, which are determinations of the Divine worship; and "judicial" precepts, which are determinations of the justice to be maintained among men. Wherefore the Apostle (Rom. 7:12) after saying that the "Law is holy," adds that "the commandment is just, and holy, and good": "just," in respect of the judicial precepts; "holy," with regard to the ceremonial precepts (since the word "sanctus"---"holy"---is applied to that which is consecrated to God); and "good," i.e. conducive to virtue, as to the moral precepts.
The
Westminster Confession of Faith later broke the law into those same categories. It says this in chapter 19, "Of the Law of God":
I. God gave to Adam a law, as a covenant of works, by which he bound him and all his posterity to personal, entire, exact, and perpetual obedience; promised life upon the fulfilling, and threatened death upon the breach of it; and endued him with power and ability to keep it.
II. This law, after his Fall, continued to be a perfect rule of righteousness; and, as such, was delivered by God upon mount Sinai in ten commandments, and written in two tables; the first four commandments containing our duty toward God, and the other six our duty to man.
III. Besides this law, commonly called moral, God was pleased to give to the people of Israel, as a Church under age, ceremonial laws, containing several typical ordinances, partly of worship, prefiguring Christ, his graces, actions, sufferings, and benefits; and partly holding forth divers instructions of moral duties. All which ceremonial laws are now abrogated under the New Testament.
IV. To them also, as a body politic, he gave sundry judicial laws, which expired together with the state of that people, not obliging any other, now, further than the general equity thereof may require.
V. The moral law doth forever bind all, as well justified persons as others, to the obedience thereof; and that not only in regard of the matter contained in it, but also in respect of the authority of God the Creator who gave it. Neither doth Christ in the gospel any way dissolve, but much strengthen, this obligation.
VI. Although true believers be not under the law as a covenant of works, to be thereby justified or condemned; yet is it of great use to them, as well as to others; in that, as a rule of life, informing them of the will of God and their duty, it directs and binds them to walk accordingly; discovering also the sinful pollutions of their nature, hearts, and lives; so as, examining themselves thereby, they may come to further conviction of, humiliation for, and hatred against sin; together with a clearer sight of the need they have of Christ, and the perfection of his obedience. It is likewise of use to the regenerate, to restrain their corruptions, in that it forbids sin, and the threatenings of it serve to show what even their sins deserve, and what afflictions in this life they may expect for them, although freed from the curse thereof threatened in the law. The promises of it, in like manner, show them God's approbation of obedience, and what blessings they may expect upon the performance thereof; although not as due to them by the law as a covenant of works: so as a man's doing good, and refraining from evil, because the law encourageth to the one, and deterreth from the other, is no evidence of his being under the law, and not under grace.
VII. Neither are the forementioned uses of the law contrary to the grace of the gospel, but do sweetly comply with it: the Spirit of Christ subduing and enabling the will of man to do that freely and cheerfully, which the will of God, revealed in the law, requireth to be done.
In chapter 21, "Of Religious Worship and the Sabbath-day," it says this:
VII. As it is of the law of nature, that, in general, a due proportion of time be set apart for the worship of God; so, in his Word, by a positive, moral, and perpetual commandment, binding all men in all ages, he hath particularly appointed one day in seven for a Sabbath, to be kept holy unto him: which, from the beginning of the world to the resurrection of Christ, was the last day of the week; and, from the resurrection of Christ, was changed into the first day of the week, which in Scripture is called the Lord's Day, and is to be continued to the end of the world as the Christian Sabbath.
VIII. This Sabbath is to be kept holy unto the Lord when men, after a due preparing of their hearts, and ordering of their common affairs beforehand, do not only observe an holy rest all the day from their own works, words, and thoughts about their worldly employments and recreations; but also are taken up the whole time in the public and private exercises of his worship, and in the duties of necessity and mercy.
Protestants historically under the influence of the Westminster Confession were following in the footsteps of Rome, and 19th-century American Christians were steeped in the same tradition. The Adventist pioneers inherited a Catholic view of the law without even realizing it. They tried to correct the false teaching that the Sabbath had been transferred to Sunday, but they failed to understand that the whole premise behind that teaching was false as well. However, the Westminster view of the law is much less common among Protestant Christians today than it was during EGW's time, which renders her prophetic scenario rather unlikely.