Thomas Aquinas, Compendium of Theology

In his intellectual nature man resembles the angels, who are capable of sin, as also man is. We spoke of this above. Hence all that has been set forth about the punishment or glory of souls should be understood also of the glory of good angels and the punishment of bad angels. Men and angels exhibit only one point of difference in this regard: the wills of human souls receive confirmation in good or obstinacy in evil when they are separated from their bodies, as was said above; whereas angels were immediately made blessed or eternally wretched as soon as, with full deliberation of will, they fixed upon God or some created good as their end. The variability found in human souls can be accounted for, not only by the liberty of their wills, but also by the modifications their bodies undergo; but in the angels such variability comes from the freedom of will alone. And so angels achieve immutability at the very first choice they make; but souls are not rendered immutable until they leave their bodies.

To express the reward of the good, we say in the Creed: "I believe . . . in life everlasting." This life is to be understood as eternal not because of its duration alone, but much more because it is the fruition of eternity. Since in this connection there are proposed for our belief many other truths that concern the punishments of the damned and the final state of the world, the Creed of the Fathers sums up the whole doctrine in this proposition: "I look for . . . the life of the world to come." This phrase, "the world to come," takes in all these points.