7 On this account, too, did the Lord assert that the kingdom of heaven was the portion of “the violent;” and He says, “The violent take it by force;” that is, those who by strength and earnest striving are on the watch to snatch it away on the moment. On this account also Paul the Apostle says to the Corinthians, “Do you not know, that they who run in a racecourse, do all indeed run, but one receives the prize? So run, that you may obtain. Every one also who engages in the contest is temperate in all things: now these men [do it] that they may obtain a corruptible crown, but we an incorruptible.
But I so run, not as uncertainty; I fight, not as one beating the air; but I make my body livid, and bring it into subjection, lest by any means, when preaching to others, I may myself be rendered a castaway.” This able wrestler, therefore, exhorts us to the struggle for immortality, that we may be crowned, and may deem the crown precious, namely, that which is acquired by our struggle, but which does not encircle us of its own accord (sed non ultro coalitam). And the harder we strive, so much is it the more valuable; while so much the more valuable it is, so much the more should we esteem it.
And indeed those things are not esteemed so highly which come spontaneously, as those which are reached by much anxious care. Since, then, this power has been conferred upon us, both the Lord has taught and the apostle has enjoined us the more to love God, that we may reach this [prize] for ourselves by striving after it. For otherwise, no doubt, this our good would be [virtually] irrational, because not the result of trial. Moreover, the faculty of seeing would not appear to be so desirable, unless we had known what a loss it were to be devoid of sight; and health, too, is rendered all the more estimable by an acquaintance with disease; light, also, by contrasting it with darkness; and life with death.
Just in the same way is the heavenly kingdom honourable to those who have known the earthly one. But in proportion as it is more honourable, so much the more do we prize it; and if we have prized it more, we shall be the more glorious in the presence of God. The Lord has therefore endured all these things on our behalf, in order that we, having been instructed by means of them all, may be in all respects circumspect for the time to come, and that, having been rationally taught to love God, we may continue in His perfect love: for God has displayed long-suffering in the case of man's apostasy; while man has been instructed by means of it, as also the prophet says, “Your own apostasy shall heal you;” God thus determining all things beforehand for the bringing of man to perfection, for his edification, and for the revelation of His dispensations, that goodness may both be made apparent, and righteousness perfected, and that the Church may be fashioned after the image of His Son, and that man may finally be brought to maturity at some future time, becoming ripe through such privileges to see and comprehend God.
Source: Adversus haereses (New Advent)