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." (5) 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," (6) fortified and armed with the same desire and the same arms as He who, "having joy set before him, endured the cross." (7)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"(8)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." (9) 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 baptised 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." (10) 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.
Source: Exeunte Iam Anno (Vatican.va)