1 That virtue of the mind which is called Patience, is so great a gift of God, that even in Him who bestows the same upon us, that, whereby He waits for evil men that they may amend, is set forth by the name of Patience, [or long-suffering.] So, although in God there can be no suffering, and “patience” has its name a patiendo, from suffering, yet a patient God we not only faithfully believe, but also wholesomely confess. But the patience of God, of what kind and how great it is, His, Whom we say to be impassible, yet not impatient, nay even most patient, in words to unfold this who can be able?
Ineffable is therefore that patience, as is His jealousy, as His wrath, and whatever there is like to these. For if we conceive of these as they be in us, in Him are there none. We, namely, can feel none of these without molestation: but be it far from us to surmise that the impassible nature of God is liable to any molestation. But like as He is jealous without any darkening of spirit, angry without any perturbation, pitiful without any pain, repents Him without any wrongness in Him to be set right; so is He patient without anything of passion. Now therefore as concerning human patience, which we are able to conceive and beholden to have, of what sort it is, I will, as God grants and the brevity of the present discourse allows, essay to set forth.
Source: On Patience (New Advent)