Additionalism is the belief that partial detachment from the visible world aids fulfillment.
"The good of human nature is threefold. First, there are the principles of which nature is constituted, and the properties that flow from them, such as the powers of the soul, and so forth. Secondly, since man has from nature an inclination to virtue, as stated above (I-II:60:1; I-II:63:1), this inclination to virtue is a good of nature. Thirdly, the gift of original righteousness, conferred on the whole of human nature."
Thomas Aquinas, Summa Theologiae 2:1:85:1, c.1274. (Nature confers an inclination to virtue while grace confers something additional.)
Integralism is the belief that even partial detachment from the visible world hinders fulfillment.
"No single piece of our mental world is to be hermetically sealed off from the rest."
Abraham Kuyper, Inaugural Lecture, 1880.
Additionalism advocates partial detachment from this visible world combined with some degree of engagement with it, that is, division of effort between this world and the otherworld. This entails schizodoulia (divided service), dividing life into two areas, the spiritual (or religious) and the mundane (or ordinary, or secular).
Additionalist theory arises from the Scholastic view of nature and grace, first elaborated by Thomas Aquinas. He distinguishes moral virtues (prudence, justice, temperance, courage) from infused ("theological") virtues (faith, hope, charity), which he calls an "added gift" (donum superadditum).
Francis Schaeffer well characterised Additionalism by calling its otherworld an "upper storey".
Integralism affirms the integrity of our God-given task.
Integralism is not Integrationism, in fact they are incompatible. Integralism requires separation from God's foes, while Integrationism promotes integration with those foes, thus undermining integrity. I hope at some point to find a better word for one of these.
Kuyper's is the earliest clear statement of Integralism I have found. Calvin was probably an Integralist, maybe also Luther, but I think the matter was not highlighted at the time, and certainly within a generation both branches of Magisterial Protestantism were solidly Additionalist. Later, in response to the emergence of Marxism, which claimed, in a kind of counter-Intregralism of its own, to be relevant to the whole of life, certain Dutchmen began to formulate an equally far-reaching Christian stance, based on that of Calvin but more tolerant, albeit still Statist. The father of this "neo-Calvinist" movement was Guillaume Groen van Prinsterer (1801-1876). He was followed by Kuyper (1840-1920), editor of a leading Dutch daily newspaper, founder of the Free University of Amsterdam, and Prime Minister of the Netherlands. The political party associated with this movement was initially (under Groen) called "Christian Historical" and later (under Kuyper) "Antirevolutionary". Herman Dooyeweerd (1894-1977) developed a "Philosophy of the Law-Idea" in which Kuyper's ideas evolved into something like a complete Ontology and Cosmology.
Back to Otherworldalism.