But the head of every good endeavor and the guiding force of right actions is perseverance in prayer. By means of it we can daily obtain the rest of the virtues by asking God for them. By this means are engendered in those deemed worthy the fellowship of God’s holiness and of spiritual energy and the attachment of the mind disposed toward the Lord in ineffable love. For the person who daily forces himself to persevere in prayer is enflamed with divine passion and fiery desire from a spiritual love toward God, and he receives the grace of the sanctifying perfection of the Spirit.
-St. Makarios the Great, Homilies 40.2, in Spiritual Homilies, 4th century