31 Very truly, the sacraments and the sacrifice of the altar, being Christ's own actions, must be held to be capable in themselves of conveying and dispensing grace from the divine Head to the members of the Mystical Body. But if they are to produce their proper effect, it is absolutely necessary that our hearts be properly disposed to receive them. Hence the warning of Paul the Apostle with reference to holy communion, "But let a man first prove himself; and then let him eat of this bread and drink of the chalice." This explains why the Church in a brief and significant phrase calls the various acts of mortification, especially those practiced during the season of Lent, "the Christian army's defenses." They represent, in fact, the personal effort and activity of members who desire, as grace urges and aids them, to join forces with their Captain - "that we may discover . . . in our Captain," to borrow St. Augustine's words, "the fountain of grace itself." But observe that these members are alive, endowed and equipped with an intelligence and will of their own. It follows that they are strictly required to put their own lips to the fountain, imbibe and absorb for themselves the life-giving water, and rid themselves personally of anything that might hinder its nutritive effect in their souls. Emphatically, therefore, the work of redemption, which in itself is independent of our will, requires a serious interior effort on our part if we are to achieve eternal salvation.
Source: Mediator Dei (Vatican.va)