Recollecting your request and my promise, that…I would write you something on the subject of prayer to God, I feel it my duty now to discharge this debt, and in the love of Christ to minister to the satisfaction of your pious desire.
[…] What could be more suitably the business of your widowhood than to continue in supplications night and day, according to the apostle’s admonition, “She that is a widow indeed, and desolate, trusteth in God, and continueth in supplications night and day”?
It might, indeed, appear wonderful that solicitude about prayer should occupy your heart and claim the first place in it, when you are, so far as this world is concerned, noble and wealthy, and the mother of such an illustrious family, and, although a widow, not desolate, were it not that you wisely understand that in this world and in this life the soul has no sure portion.
Wherefore He who inspired you with this thought is assuredly doing what He promised to His disciples when they were grieved, not for themselves, but for the whole human family, and were despairing of the salvation of any one, after they heard from Him that it was easier for a camel to go through the eye of a needle than for a rich man to enter into the kingdom of heaven.
He gave them this marvellous and merciful reply: “The things which are impossible with men are possible with God.” He, therefore, with whom it is possible to make even the rich enter into the kingdom of heaven, inspired you with that devout anxiety which makes you think it necessary to ask my counsel on the question how you ought to pray.
For while He was yet on earth, He brought Zaccheus, though rich, into the kingdom of heaven, and, after being glorified in His resurrection and ascension, He made many who were rich to despise this present world, and made them more truly rich by extinguishing their desire for riches through His imparting to them His Holy Spirit.
For how could you desire so much to pray to God if you did not trust in Him? And how could you trust in Him if you were fixing your trust in uncertain riches, and neglecting the wholesome exhortation of the apostle:
“Charge them that are rich in this world that they be not high-minded, nor trust in uncertain riches, but in the living God, who giveth us richly all things to enjoy; that they do good, that they be rich in good works, ready to distribute, willing to communicate, laying up in store for themselves a good foundation, that they may lay hold on eternal life”?
Augustine of Hippo (354-430): Letter to Proba, I, 1-2 @ Crossroads Initiative.