37 Union in prayer and in active participation in the mysteries of the sacred liturgy enormously enriches and completes the Christian life of individuals and of the whole community, and it greatly helps educate the soul to charity, which is the distinguishing mark of the true Christian; a charity, We say, which overcomes all differences between languages and nationalities, and amicably embraces all men, whether brothers or enemies. In this connection, We like to repeat the words of Our predecessor Pope Clement: "When they (the pagans) hear from us that God says, 'You have no merit if you love those who love you, but you have merit if you love your enemies and those who hate you' (65)—when they hear this, they admire the grace of your charity; but when they see that we not only do not love those who hate us, but do not even reciprocate the love of those who love us, they will mock us and God's name will be blasphemed." (66) The greatest missionary of all, St. Paul the Apostle, at the time when he was on the point of bringing the message of God's word to the people, as far as the farthest reaches of the Western world, wrote to the Romans and exhorted them to practice "love without pretense." (67) Earlier, with sublime expression, he had praised that virtue—without which a Christian is nothing. (68)
Source: Princeps Pastorum (Vatican.va)