Treatment of the concept
Holiness (A. S. hal, perfect, or whole). Sanctitas in the Vulgate of the New Testament is the rendering of two distinct words, aliosune (I Thess., iii, 13) and osiotes (Luke, i, 75; Eph., iv, 24). These two Greek words express respectively the two ideas connoted by "holiness" viz.: that of separation as seen in alos from alos, which denotes "any matter of religious awe" (the Latin sacer); and that of sanctioned (sancitus), that which is osios, has received God's seal. Considerable confusion is caused by the Reims version which renders aliasmos by "holiness" in Heb., xii, 14, but more correctly elsewhere by "sanctification", while aliosune, which is only once rendered correctly "holiness", is twice translated "sanctification".
St. Thomas (I-II, Q. lxxxi, art. 8) insists on the two aspects of holiness mentioned above, viz., separation and firmness, though he arrives at these meanings by dint of the etymologies of Origen and St. Isidore. Sanctity, says the Angelic Doctor, is the term used for all that is dedicated to the Divine service, whether persons or things. Such must be pure or separated from the world, for the mind must needs be withdrawn from the contemplation of inferior things if it is to be set upon the Supreme Truth—and this, too, with firmness or stability, since it is a question of attachment to that which is our ultimate end and primary principle, viz., God Himself—"I am sure that neither death, nor life, nor angels ... nor any other creature shall be able to separate us from the love of God" (Rom., viii, 38-39). Hence St. Thomas defines holiness as that virtue by which a man's mind applies itself and all its acts to God; he ranks it among the infused moral virtues, and identifies it with the virtue of religion, but with this difference that, whereas religion is the virtue whereby we offer God due service in the things which pertain to the Divine service, holiness is the virtue by which we make all our acts subservient to God. Thus holiness or sanctity is the outcome of sanctification, that Divine act by which God freely justifies us, and by which He has claimed us for His own; by our resulting sanctity, in act as well as in habit, we claim Him as our Beginning and as the End towards which we daily unflinchingly tend. Thus in the moral order sanctity is the assertion of the paramount rights of God; its concrete manifestation is the keeping of the Commandments, hence St. Paul: "Follow peace with all men, and holiness [sanctimoniam, aliasmon]: without which no man shall see God" (Heb., xii, 14). The Greek word should be noted; it is generally rendered "sanctification", but it is noteworthy that it is the word chosen by the Greek translators of the Old Testament to render the Hebrew word `Z), which properly means strength or stability, a meaning which as we have seen is contained in the word holiness. Thus to keep the Commandments faithfully involves a very real though hidden separation from this world, as it also demands a great strength of character or stability in the service of God.
It is manifest, however, that there are degrees in this separation from the world and in this stability in God's service. All who would serve God truly must live up to the principles of moral theology, and only so can men save their souls. But others yearn for something higher; they ask for a greater degree of separation from earthly things and a more intense application to the things of God. In St. Thomas's own words: "All who worship God may be called `religious', but they are specially called so who dedicate their whole lives to the Divine worship, and withdraw themselves from worldly concerns, just as those are not termed 'contemplatives' who merely contemplate, but those who devote their whole lives to contemplation". The saint adds: "And such men subject themselves to other men not for man's sake but for God's sake", words which afford us the keynote of religious life strictly so-called (II-II, Q. lxxxi, a. 7, ad 5um)