Saint Macarius the Great
That we need a great zeal for prayer, and that the Spirit who dwells in our spirits brings it about, is clearly shown by the words of the Apostle, who exhorts us: "Pray all the time, asking for what you need, praying in the Spirit on every possible occasion" (Eph 6:18). Whoever of the brothers dedicates himself to this, namely, unceasing prayer, possesses a beautiful treasure, which becomes the greatest possession of one who loves God with a firm and right conscience." He never willingly strays away into distractions, nor does he have an unwonted debt that needs paying; but rather he fulfills the love and desires of his spirit, displaying to all the brothers the good fruits that come from perseverance. It is necessary, however, that the others in the community also make time for such unceasing prayer and rejoice in persevering in prayer so that they become sharers of such a life. The Lord himself will surely give to those who ask how they must pray, according to what is said: "He gives to him what he has asked for" (Ps 106:15).
Therefore, we ought to ask and to know that the better one makes an effort in the work of prayer, the more he must sustain the battle with great care and all virtue. For great battles demand great labors because evil insidiously rises up from all sides. Such obstacles the devil places in his path to impede his diligence, such as sleep, listlessness, physical torpor, distraction of thought, confusion of intellect, debility, and other evil passions and workings to kill the soul that is partly torn away and given over to its enemy. Therefore, it is necessary, as a wise ship captain does, that one in prayer controls the spirit and never gives the mind over to the turbulent suggestions of the evil spirit, nor allows it to be tossed about by waves. He must keep his eyes directed on the heavenly port and offer himself to God, who has believed in him and wants that from him.
Nor should they in prayer be satisfied with merely standing or kneeling, seemingly to be agreeing with Scripture and well-pleasing to God, while all the while their mind wanders far from him. They must guard against every neglect of thoughts and unseemly attitude and turn the whole soul with the body back to prayer. And the superiors also ought to assist such a person, and with every solicitude and correction to nurture the one who is learning how to pray with fervor according to what is outlined for him. They should also diligently purify his soul. For those who so conduct themselves, let them share the fruit of their virtues even with the weak, not only to him who already is growing in perfection, but also with children and those uneducated in doctrine, consoling them and arousing them to imitate what they see in such advanced teachers.
Fruit of True Prayer
The fruits of sincere prayer are simplicity, love, humility, fortitude, innocence, and other things similar to these. Such fruits which precede the heavenly fruit are developed in this life by a man eager for prayer through hard labor. Prayer is adorned with such fruit. Who lacks such fruit undertakes in vain laborious tasks. This applies not only to prayer, but also to every path of philosophy, which is born out of such a growth process. It is truly a way of justice and leads to the proper goal. One who is lacking in these is left only with vanity and is similar to the foolish virgins, who did not have at the necessary time the spiritual oil to enter into the wedding feast (Mt 25:1-13). For such did not have a light in their hearts, the fruit of virtue, nor the light of the Spirit in their souls. This is why Scripture rightly called them foolish, because they lacked virtue before the coming of the Bridegroom and were abjectly excluded from the heavenly bridal chamber. They did not enjoy any reward for their efforts of virginity because the power of the Spirit was not with them. When we cultivate a vineyard, the whole of our attention and labor is given in the expectation of the harvest. If there is no vintage, all our work is to no purpose. Similarly, if through the activity of the Spirit we do not perceive within ourselves the fruits of love, peace, and the other qualities mentioned by St. Paul (Gal 5:22), then our labor for the sake of virginity, prayer, psalmody, fasting, and vigil is useless.
From the Great Letter in Pseudo-Macarius: The Spiritual Homilies and the Great Letter, Paulist Press, pp 267-268
Read also Saint Macarius' Homilies 19 and 33.