God's grace is the outcome of His mercy. Now both are said in Psalm 58:11: "His mercy shall prevent me," and again, Psalm 22:6: "Thy mercy will follow me." Therefore grace is fittingly divided into prevenient and subsequent.
....As grace is divided into operating and cooperating, with regard to its diverse effects, so also is it divided into prevenient and subsequent, howsoever we consider grace. Now there are five effects of grace in us: of these, the first is, to heal the soul; the second, to desire good; the third, to carry into effect the good proposed; the fourth, to persevere in good; the fifth, to reach glory. And hence grace, inasmuch as it causes the first effect in us, is called prevenient with respect to the second, and inasmuch as it causes the second, it is called subsequent with respect to the first effect. And as one effect is posterior to this effect, and prior to that, so may grace be called prevenient and subsequent on account of the same effect viewed relatively to divers others. And this is what Augustine says (De Natura et Gratia xxxi): "It is prevenient, inasmuch as it heals, and subsequent, inasmuch as, being healed, we are strengthened; it is prevenient, inasmuch as we are called, and subsequent, inasmuch as we are glorified."