Catholics often speak of the “infusion” of grace. Protestants are often allergic to this language, perceiving it to be a threat to the legal status of our justification. But in fact, Protestants also believe in the “infusion” of grace, and some Protestant theologians (read: the brightest ones) are not shy to speak this way (e.g. Jonathan Edwards).
What do Catholics mean when they speak of “infusion”? That’s like asking what Protestants mean when they speak of God’s “giving” grace; it all depends on which Protestant you talk to; there are likely ten different answers for every ten theologians answering. However, a certain continuity can easily be found in the Catholic ways of speaking about “infusion” just as a certain continuity can be found in Protestants who talk about “giving [of grace].”
No theologian influences Catholic ways of theological language more, probably, than St. Thomas Aquinas. What does Aquinas mean when he speaks of “infusion”? For example, Aquinas believes that charity (love for God) is a divine gift of the Holy Spirit that is “infused” into us. What does he mean? Here is a few small excerpts from his writings I believe partly illuminate an answer to this question.
[Charity] is not founded principally on the virtue of a man, but on the goodness of God. ST II-II.23.3.ad.1
Charity is superior to the soul, in as much as it is a participation of the Holy Ghost. ST II-II.23.3.ad.3
The infusion of charity denotes a change to the state of having charity from the state of not having it, so that something must needs come which was not there before. On the other hand, the increase of charity denotes a change to more having from less having, so that there is need, not for anything to be there that was not there before, but for something to be more there that previously was less there. This is what God does when He increases charity, that is He makes it to have a greater hold on the soul, and the likeness of the Holy Ghost to be more perfectly participated by the soul. ST II-II.24.5.ad.3
Here Aquinas distinguishes between infusion and increase. God infuses charity instantaneously (from not having to having is like from not-pregnant to pregnant), and this is different from our increase in charity. Our increase in charity does have a similarity to infusion, however, for according to the Doctor, God is the one who works both in us.