A system of morality based exclusively on human reason robs man of his highest dignity and lowers him from the supernatural to the merely natural life. Not but that man is able by the right use of reason to know and to obey certain principles of the natural law. But though he should know them all and keep them inviolate through life-and even this is impossible without the aid of the grace of our Redeemer-still it is vain for anyone without faith to promise himself eternal salvation. ‘If anyone abide not in Me, he shall be cast forth as a branch, and shall wither, and they shall gather him up and cast him into the fire, and he burneth’ (John XV., 6). ‘He that believeth not shall be condemned’ (Mark XVI., 16).

–  Leo XIII, Tametsi Futura Prospicientibus 11

It would be a very serious error to conclude that the Church’s teaching is essentially only an ‘ideal’ which must then be adapted, proportioned, graduated to the so-called concrete possibilities of man, according to a ‘balancing of the goods in question’. But what are the ‘concrete possibilities of man’? And of which man are we speaking? Of man dominated by lust or of man redeemed by Christ? This is what is at stake: the reality of Christ’s redemption. Christ has redeemed us! This means that he has given us the possibility of realizing the entire truth of our being; he has set our freedom free from the domination of concupiscence. And if redeemed man still sins, this is not due to an imperfection of Christ’s redemptive act, but to man’s will not to avail himself of the grace which flows from that act. God’s command is of course proportioned to man’s capabilities; but to the capabilities of the man to whom the Holy Spirit has been given; of the man who, though he has fallen into sin, can always obtain pardon and enjoy the presence of the Holy Spirit

– St John Paul II, Veritatis Splendor 103