Eternal life is the common inheritance of every soul. To each, in virtue of its spiritual nature, is annexed immortality as a participation in the prerogatives of God. The conditions of that life may differ vitally, in that for some it is to be a reward and for others a punishment; but though in this way it is evident that those who are possessed of it may be very variously placed (even such as are rewarded by the vision of God's face being capable of unequal measures of knowledge and love), yet it is eternity as some sort of duration that is to be the same for both. To grasp it may be beyond the power of man's imagination, but the intellect can formulate truths about it without necessarily pretending to comprehend them. But of this much the reason can take cognisance - namely, that the eternity promised to man is the free gift of God. The idea of annihilation, repugnant as it seems to be to human nature, is a course that is in the abstract possible to God; for the whole basis of the spiritual life supposes that the divine power was exercised, not only to call us into being, but is continually being exercised to keep us in existence. Nor is this need of being upheld caused in any way by the fall of the race. It is no punishment for sin that we have to be supported by the Hands of God - it is rather a consequence simply of our position as creatures. Just because He alone is the uncaused existence, He alone exists of Himself; and annihilation, therefore, is averted, and immortality secured, by the free promise and fulfilment of God.
Definitions of this eternity are various, and from the nature of the case must needs be inadequate, but eternity may be best described as a persistent "Now." Just as for those whose attention is very steadily fixed on some absorbing matter, time has no meaning and the seconds and minutes slip by unnoticed, so in the absorbing interest of eternal happiness or pain there can be no idea of the passage of time. This is taught by those old legends that have so caught the fancy of many poets, of monks hearing the song of a bird of such entrancing loveliness that when they returned to the cloister they found that a hundred years had elapsed since they had begun to listen to the music. There are tales even more descriptive in the pages of mere psychologists, where the same idea of the riveting effect of attention is steadily insisted on. The whole idea, therefore, of duration has been transferred from the mere notion of the distance between passing moments into the perhaps vaguer notion of fluidity, movement, change. Now in eternity we assert that all "the dull monotonies of change" will have ceased altogether and life will be definable not in terms of growth but only as sublime consciousness. Perhaps the very idea of eternity having no end gives the unpleasant sense of boredom with which certain people seem to regard the notion of life everlasting, and has subjected it to light raillery. No past is possible, no future to be expected, because of the very absorption in the present - that is all we mean.
Perhaps this will help me to understand what is the difficulty in the idea of heaven and hell. Fling into eternity a man who dies in revolt against God, fling him therefore into an absorbing idea of hatred, and what possibility is there for his repentance or ultimate forgiveness? It would be as absurd as for one who was engaged in the vision of God to fall from the love of God. Quite sincerely we can say, in the full meaning of the expression, that there's not the time either in Heaven or Hell to do so. There is no time at all, no change, but a ceaseless "Now." There can be no future. Even the soul in Purgatory does not suffer change in its ultimate destination. As it crosses the threshold of the next life, it knows its final reward and can no longer be turned from the love of its Maker. Thenceforward there is no merit and no demerit; no increase can be made in the volume of divine love, only the fettering chains broken, the stains washed clean, the debts fully paid. For me shall be one day this changeless existence, this everlasting "Now." Shall its persistence be joy or pain? In this life of time I must make up my mind and fix my duty, for there I can neither repent nor fall. But the point that needs most insistence is the consideration of what precisely is meant by the ceaseless "Now." Just as, when my attention is taken up with anything that absorbs my interest, all notion of passing time is lost to me, so the meaning of the eternity of Heaven is made intelligible to me on Earth.
- text taken from Meditations for Layfolk by Father Bede Jarrett, O.P.