Never believe that you have acquired a virtue, if you have not made proof of it in resisting its contrary vice, and unless you practise it faithfully on suitable occasions. -St. Teresa
St. Vincent de Paul was not contented, as so many are, with knowing and loving virtues, but he applied himself continually to the practice of them. It was his maxim, that labor and patience are the best means of acquiring and planting them firmly in our hearts, and that virtues acquired without effort or difficulty can be easily lost, while those which have been beaten by the storms of temptation, and practised amid the difficulties and repugnances of nature, sink their roots deep into the heart. And so, on such occasions, instead of being sad, he appeared unusually cheerful.
By this same sentiment, St. Philip Neri encouraged his penitents not to grieve when they suffer temptations and trials, telling them that when the Lord intends to confer on any one some particular virtue, He is accustomed to permit him to be first assailed by the contrary vice.
Text from A Year With the Saints (New York: P.J. Kenedy & Sons, 1891).