John Cassian was born about 360 in Dobrogea , located in the Danube delta in what is modern Romania. Somewhat atypically for an Eastern Christian, his native language was Latin. He entered a Bethlehem monastery in his early twenties but spent many years as a visitor among the desert fathers in Egypt. He next shows up in Constantinople, where he was ordained a deacon by St John Chrysostom. When the latter was deposed, Cassian had to leave and ended up near Marseilles, where he became a priest and founded one monastery for men and another for women. He died in 435.

His works are the Institutes and the Conferences, from which latter work (New York, Paulist Press, 1985) the following quotes are taken. Although not recognized as a saint by the Western church, he is mentioned in Benedict's Rule and his feast is locally celebrated in Marseilles on today's date.

Prayer changes at every moment in proportion to the degree of purity in the soul and in accordance with the extent to which the soul is moved either by outside influence or of itself. Certainly the same kind of prayers cannot be uttered continuously by by any one person. A lively person prays one way. A person brought down by the weight of gloom or despair prays another. One prays another way when the life of the spirit is flourishing, and another way when pushed down by the mass of temptation. One prays differently, depending on whether one is seeking the gift of some grace or virtue or the removal of some sinful vice. The prayer is different once again when one is sorrowing at the thought of hell and the fear of future judgement, or when one is fired by hope and longing for future blessedness, when one is in need or peril, in peace or tranquility, when one is flooded with the light of heavenly mysteries or when one is hemmed in by aridity in virtue and staleness in one's thinking. (p. 107)

We need to be especially careful to follow the gospel precept which instructs us to go into our room and to shut the door so that we may pray to our Father. And this is how we can do it.

We pray in our room whenever we withdraw our hearts completely from the tumult and the noise of our thoughts and our worries and when secretly and intimately we offer our prayers to the Lord.

We pray with the door shut when without opening our mouths and in perfect silence we offer our petitions to the One who pays no attention to words but who looks hard at our hearts.

We pray in secret when in our hearts alone and in our recollected spirits we address God and reveal our wishes only to Him and in such a way that the hostile powers themselves have no inkling of their nature. Hence we must pray in utter silence, not simply in order that our whispers and our cries do not prove both a distraction to our brothers standing nearby and a nuisance to them when they themselves are praying but also so as to ensure that the thrust of our pleading be hidden from our enemies who are especially lying in wait to attack us during our prayers. In this way we shall fulfill the command "Keep your mouth shut from the one who sleeps on your breast" (Mi 7:5).

The reason why our prayers ought to be frequent and brief is in case the enemy, who is out to trap us, should slip a distraction to us if ever we are long-drawn-out. There lies true sacrifice. "The sacrifice which God wants is a contrite heart" (Ps 50:19). This indeed is the saving oblation, the pure offering, the sacrifice of justification, the sacrifice of praise. These are the real and rich thank offerings, the fat holocausts offered up by contrite and humble hearts. If we offer them to God in the way and with zeal which I have mentioned we can be sure to be heard and we can sing: "Let my prayer rise up like incense before your face and my hands like the evening offering" (Ps 140:2). (p. 123-124).