Saint Peter of Damaskos. Book1. A Treasury of Divine Knowledge. From Philokalia, Vol. 3.
If by the grace of God you have received the gift of discrimination, you should in great humility do everything you can to guard it, so that you do nothing without it. Otherwise you will bring on yourself greater chastisement by sinning knowingly because of your negligence. If you have not received this gift you should not think, say or do anything without consulting others about it, and without a basis of firm faith and pure prayer. Without such faith and such prayer you will never truly achieve discrimination.
Discrimination is born of humility. On its possessor it confers spiritual insight, as both Moses and St John Klimakos say: such a man foresees the hidden designs of the enemy and foils them before they are put into operation. It is as David states: ‘And my eyes looked down upon my enemies’ (Ps. 14:7. LXX). Discrimination is characterized by an unerring recognition of what is good and what is not, and the knowledge of the will of God in all that one does. Spiritual insight is characterized, first, by awareness of one’s own failings before they issue in outward actions, as well as of the stealthy tricks of the demons; and, second, by the knowledge of the mysteries hidden in the divine Scriptures and in sensible creation.
As has been already explained, humility, the mother of discrimination and spiritual insight, likewise has its own characteristic by which it is known. The humble person must possess every virtue and yet truly think himself the greatest of debtors and inferior to everything else in creation. If, however, a person does not think in this way, then he can be assured that he is in fact inferior to everything else in creation, even though he seems to lead a life like that of the angels. For even a true angel possessing so many virtues and so much wisdom cannot conform to the Creator’s will unless he also possesses humility. What, then, can a person who thinks that he is an angel say for himself if he lacks humility, source of all present and future blessings, begetter of that discrimination which illumines the ends of the earth and without which all things are obscure?
Discrimination is not only called light; it truly is light. We need this light before we say or do anything. When it is present we are able to view everything else with wonder. We can marvel at how God, on the first and greatest of days, began by creating light, so that what was subsequently created might not he invisible and as if it did not exist, as St John of Damaskos says.’ Let it be said again: discrimination is light; and the spiritual insight it generates is more necessary than all other gifts. For what is more necessary than to perceive the wiles of the demons and with the help of God’s grace to protect one’s soul? Other things most necessary to us include, according to St Isaac, purity of conscience and, according to the apostle, the sanctification of the body (cf. Rom. 12:1; 1 Cor. 6:19-20) without which ‘no one will see the Lord’ (Heb. 12:14).