The following is taken from the encyclical Exeunte Iam Anno.
10. Now the whole essence of a Christian life is to reject the corruption of the world and to oppose constantly any indulgence in it; this is taught in the words and deeds, the laws and institutions, the life and death of Jesus Christ, "the author and finisher of faith." Hence, however strongly We are deterred by the evil disposition of nature and character, it is our duty to run to the "fight proposed to Us," fortified and armed with the same desire and the same arms as He who, "having joy set before him, endured the cross." Wherefore let men understand this specially, that it is most contrary to Christian duty to follow, in worldly fashion, pleasures of every kind, to be afraid of the hardships attending a virtuous life, and to deny nothing to self that soothes and delights the senses. "They that are Christ's, have crucified their flesh, with the vices and concupiscences" -- so that it follows that they who are not accustomed to suffering, and who hold not ease and pleasure in contempt belong not to Christ. By the infinite goodness of God man lived again to the hope of an immortal life, from which he had been cut off, but he cannot attain to it if he strives not to walk in the very footsteps of Christ and conform his mind to Christ's by the meditation of Christ's example. Therefore this is not a counsel but a duty, and it is the duty, not of those only who desire a more perfect life, but clearly of every man "always bearing about in our body the mortification of Jesus." How otherwise could the natural law, commanding man to live virtuously, be kept? For by holy baptism the sin which we contracted at birth is destroyed, but the evil and tortuous roots of sin, which sin has engrafted, and by no means removed. This part of man which is without reason -- although it cannot beat those who fight manfully by Christ's grace -- nevertheless struggles with reason for supremacy, clouds the whole soul and tyrannically bends the will from virtue with such power that we cannot escape vice or do our duty except by a daily struggle. "This holy synod teaches that in the baptized there remains concupiscence or an inclination to evil, which, being left to be fought against, cannot hurt those who do not consent to it, and manfully fight against it by the grace of Jesus Christ; for he is not crowned who does not strive lawfully." There is in this struggle a degree of strength to which only a very perfect virtue, belonging to those who, by putting to flight evil passions, has gained so high a place as to seem almost to live a heavenly life on earth. Granted; grant that few attain such excellence; even the philosophy of the ancients taught that every man should restrain his evil desires, and still more and with greater care those who from daily contact with the world have the greater temptations -- unless it be foolishly thought that where the danger is greater watchfulness is less needed, or that they who are more grievously ill need fewer medicines.
11. But the toil which is borne in this conflict is compensated by great blessings, beyond and above heavenly and eternal rewards, particularly in this way, that by calming the passions nature is largely restored to its pristine dignity. For man has been born under this law, that the mind should rule the body, that the appetites should be restrained by sound sense and reason; and hence it follows that putting a curb upon our masterful passions is the noblest and greatest freedom. Moreover, in the present state of society it is difficult to see what man could be expected to do without such a disposition. Will he be inclined to do well who has been accustomed to guide his actions by self-love alone? No man can be high-souled, kind, merciful, or restrained, who has not learnt selfconquest and a contempt for this world when opposed to virtue. And yet it must be said that it seems to have been pre-determined by the counsel of God that there should be no salvation to men without strife and pain. Truly, though God has given to man pardon for sin, He gave it under the condition that His only begotten Son should pay the due penalty; and although Jesus Christ might have satisfied divine justice in other ways, nevertheless He preferred to satisfy by the utmost suffering and the sacrifice of His life. Thus he has imposed upon His followers this law, signed in His blood, that their life should be an endless strife with the vices of the age. What made the apostles invincible in their mission of teaching truth to the world; what strengthened the martyrs innumerable in their bloody testimony to the Christian faith, but the readiness of their soul to obey fearlessly His laws? And all who have taken heed to live a Christian life and seek virtue have trodden the same path; therefore We must walk in this way if We desire either Our own salvation or that of others. Thus it becomes necessary for every one to guard manfully against the allurements of luxury, and since on every side there is so much ostentation in the enjoyment of wealth, the soul must be fortified against the dangerous snares of riches lest straining after what are called the good things of life, which cannot satisfy and soon fade away, the soul should lose "the treasure in heaven which faileth not." Finally, this is matter of deep grief, that free-thought and evil example have so evil an influence in enervating the soul, that many are now almost ashamed of the name of Christian -- a shame which is the sign either of abandoned wickedness or the extreme of cowardice; each detestable and each of the highest injury to man. For what salvation remains for such men, or on what hope can they rely, if they cease to glory in the name of Jesus Christ, if they openly and constantly refuse to mold their lives on the precepts of the gospel? It is the common complaint that the age is barren of brave men. Bring back a Christian code of life, and thereby the minds of men will regain their firmness and constancy. But man's power by itself is not equal to the responsibility of so many duties. As We must ask God for daily bread for the sustenance of the body, so must We pray to Him for strength of soul for its nourishment in virtue. Hence that universal condition and law of life, which We have said is a perpetual battle, brings with it the necessity of prayer to God. For, as is well and wisely said by St. Augustine, pious prayer flies over the world's barriers and calls down the mercy of God from heaven. In order to conquer the emotions of lust, and the snares of the devil, lest we should be led into evil, we are commanded to seek the divine help in the words, "pray that ye enter not into temptation." How much more is this necessary, if we wish to labor for the salvation of others? Christ our Lord, the only begotten Son of God, the source of all grace and virtue, first showed by example what he taught in word: "He passed the whole night in the prayer of God," and when nigh to the sacrifice of his life, "He prayed the longer."