Fate is admitted in the sense that whatever happens here below is subject to Divine Providence; ordained thereby, and, as it were, foretold. For whatever seems to occur accidentally, either in material or in human things, may be ascribed to some preordained cause, which is the Providence of God. But theologians do not, habitually, make use of this word, because fate was supposed to depend upon a disposition of the stars, which imposed a necessity upon sublunary things. This is evidently false, as has been already said; neither is there any reason why a thing should not be accidental in relation to inferior causes, although intended by a superior cause.
Therefore fate, understood causally, stands for the Providence of God; for it signifies the power of willing effects, or the Will of God: while considered as existing in secondary causes it means the disposition of these causes. In this sense it is, properly speaking, mutable, although as proceeding from Divine Providence, it is immutable.
It is a mistake, however, to conclude, as some do, that everything happens by necessity. The Egyptians were also mistaken in believing that sacrifices could alter fate; which would be contrary to the immutability of the Divine foreknowledge.
Nor is everything subject to fate, because fate is itself subject to those things to which second causes are subject; while things which are done immediately by God, such as creation, and the glorification of spiritual substances, are not subject to second causes, and therefore not subject to fate.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni