Faith is a comprehensive knowledge of the essentials, and knowledge is the strong and sure demonstration of what is received by faith, built upon faith by the Lord’s teaching, conveying the soul on to infallibility, science, and comprehension. And, in my view, the first saving change is that from heathenism to faith, and the second, that from faith to knowledge. And the latter terminating in love, thereafter gives the loving to the loved, that which knows to that which is known. — Clement of Alexandria
St. Cyril of Jerusalem: For the method of godliness consists of these two things, pious doctrines, and virtuous practice: and neither are the doctrines acceptable to God apart from good works, nor does God accept the works which are not perfected with pious doctrines. – Catechetical Lectures
Abba John Cassian related with regard to another old man living in the desert, that he had asked God to grant him never to become sleepy during a spiritual conference, but, if someone uttered slanderous or useless words, to be able to go to sleep at once, so that his ears should never be touched by that poison. This old man also said that the devil, enemy of all spiritual instruction, works hard to provoke useless words.
He used the following example, “Once when I was talking to some brothers on a helpful topic, they were overcome by sleep so deep, that they could not even move their eyelids any longer. Then, wishing to show them the power of the devil, I introduced a trivial subject of conversation. Immediately, they woke up, full of joy. Then I said to them with many sighs, ‘Until now, we were discussing heavenly things and your eyes were heavy with sleep, but when I embarked on a useless discourse, you all woke up with alacrity. Therefore, brothers, I implore you to recognize the power of the evil demon; pay attention to yourselves, and guard yourselves from the desire to sleep when you are doing or listening to something spiritual.’” — The Desert Fathers
St. Seraphim of Sarov: Before anything else one must believe in God, “that He is, and that He is a rewarder of them that diligently seek Him” (Hebrews 11:6).
— Spiritual Instructions
St. Symeon the New Theologian: Belief is a matter of dying for Christ and His commandments. It is believing that such a death is life-giving. It is to count poverty as riches, and to consider the lowest humiliation as true honor and nobility. Faith is believing that when one has nothing, one has everything. More than this, it is to possess the incomprehensible riches of the knowledge of Christ and to look upon all visible things as but clay and smoke.
— The Practical and Theological Chapters