Fortitude and perseverance…

And although many conflicts of diverse kinds should abound in prayer, and darkness of mind with much confusion, the devil making the soul feel that her prayer was not pleasing to God, nevertheless, she ought not to give up on account of those conflicts and shadows, but to abide firm in fortitude and long perseverance, considering that the devil so does to draw her away from prayer the mother, and God permits it to test the fortitude and constancy of that soul.
–Saint Catherine of Siena  (1347-1380)

The poison of temptations…

There is no man who will not be grieved at the time of his chastisement; and there is not man who will not endure a bitter time, when he must drink the poison of temptations. Without them, it is not possible to obtain a strong will. When he has often experienced the help of God in temptations, a man also obtains strong faith.
–Saint Isaac of Syria (Seventh Century)