Saints Barsanuphius and John the Prophet. "Letters. Vol. 2."
Question from the same [brother] to the same [Old man]: "Although I am slow in learning, nevertheless I am able to learn the Psalms more quickly. Tell me whether this has happened to me from God, or else from the demons in order that I might come to vainglory". Response by Barsanuphius.
Whatever God grants you, receive it with humility. For to learn the words of God easily does not actually come from the devil, but they are in fact a seed from God. If, however, one is not careful, then the devil, too, sows own tares. If you want to humble your thought, give it the following example: "If one receives from the master certain silver coins, as in [the story of] those servants [Cf. Mt 25.14], but does not make some profit or increase, then what will happen?" And your thought will respond: "Whatever happened to that servant who hid the silver of the master." [Cf. Mt 25.24-30] Then you can say to your thought: "So do not feel proud, filling the air with fruitless words, because these shall be to my condemnation."
Question from the same person to the same [Old man]: "When I do something good, how should I humble my thought? And how can I bear blame on myself when I have done something good? Response by Barsanuphius.
In order to humble your thought, when you do what is good and keep all the commandements, remember him who said: "When you have done all that you were ordered to do, say, ‘We are the worthless slaves; we have only done what we ought to have done’" [Lk 17.10] How much more so, when we have not yet reached the point of fulfilling even one commandement! This is what you must always think, as well as bearing the blame in regard to the good deed, saying: "I do not know if it will be pleasing to God." [Sayings, Agathon 29]
Doing the will of God is a great thing, but fulfilling it is an even greater thing; for it is the completion of all the commandments. Indeed, doing God’s will is only a partial deed; certainly it is less than fulfilling it. This is why the Apostle said: "Forgetting what lies behind and straining forward to what lies ahead." [Phil 3.13] So then, as long as one strains forward, there is no stopping, but one always considers oneself as lagging behind, and therefore one continually progresses. For he also said: "Those of us who are mature are of the same mind," namely, in regard to [spiritual] progress; and he added: "If you think differently about anything, this, too, the Lord will reveal to you." [Cf. Phil 3.15]