“Drink water out of your own cistern,” that is, examine your own resources, do not go to the springs belonging to others, but from your own streams gather for yourself the consolations of life. Do you have metal plates, clothing, beasts of burden, utensils of every kind? Sell them; permit all things to go except your [soul’s] liberty.
Let us return to the sacred Scriptures and “drink water from our own cisterns and running water from our own wells.” Let us drink of the living water, “springing up unto life everlasting.” … Not visible rivers merely watering the earth with its thorns and trees, but enlightening souls.
And I hope, O man, that you imitate the example of this kind, so that you yourself may bear fruit of joy and delight! The sweetness of your grace is within yourself, it sprouts from you, it remains in you, it is within you, that is, the joy of your conscience is to be sought within yourself. Therefore, it says: Drink water from your own vessels, and from the fountains of your own wells (Prov., V, 15).
“Let the fountain of your water be your own and let no stranger share with you.” For all who do not love God are strangers, are antichrists. And although they enter the basilicas, they cannot be numbered among the sons of God. That fountain of life does not belong to them. Even an evil person can have baptism; even an evil person can have prophecy. We find that king Saul had prophecy; he was persecuting the holy David and was filled with the Spirit of prophecy and began to prophesy. Even an evil person can receive the sacrament of the body and blood of the Lord, for about such it has been said, “He who eats and drinks unworthily eats and drinks judgment to himself.” Even an evil person can have the name of Christ, that is, even an evil person can be called Christian; and about these it has been said, “They profaned the name of their God.” Therefore, even an evil man can have all these mysteries. But he cannot have love and be evil. This, then, is the peculiar gift; it is the unique fountain. For drinking of this the Spirit of God encourages you; for drinking of himself the Spirit of God encourages you.
“Drink the waters from your own wells, fresh water from your own source.” … As the prophet Isaiah declares, “You will be like a well-watered garden, like a flowing spring whose waters will never fail. And places emptied for ages will be built up in you. You will lift up the foundations laid by generation after generation. You will be called the builder of fences, the one who turns the pathways toward peace.” … And so it will happen that not only the whole thrust and thought of your heart but even all the wanderings and the straying of your thoughts will turn into a holy and unending meditation on the law of God.
[AD 379] Basil of Caesarea on Proverbs 5:15-17