The term comes from the New Testament's First Epistle of Peter (5:8, , . ? ? ? ? -- NIV: Be alert and of sober mind. Your enemy the devil prowls around like a roaring lion looking for someone to devour). There nepsis appears in a verb form, in the imperative mood, as an urgent command to vigilance and awakeness: "be alert and awake".
Perhaps most associated with Orthodox monasticism, innumerable references to nepsis are made in The Philokalia (the full title of The Philokalia being The Philokalia of the Neptic Fathers). Parallels have been drawn between nepsis and Jewish devekut.
In Orthodox Christianity, the struggle against the corruption of the passions is conducted through ascetic effort to purify the soul (asceticism from Greek: askesis "exercise"). At the advanced stages this involves "bringing the mind into the heart" ("mind" is a substitution for the tricky-to-translate Greek nous (?), which here indicates that faculty of the soul by which man enters into communion with God). Purification of the soul, which is achieved only through the help of divine grace, is pursued through one's efforts to fulfill the commandments of Christ, participation in the Holy Mysteries of the Christian Orthodox Church, private prayer including devotion to the Jesus Prayer, fasting according to the Church calendar, study of Holy Scripture and the lives of the saints, and vigilant watchfulness over the thoughts to prevent sinful thoughts from becoming sinful actions, and then passions.
Greek monks have never used the teachings of Plato or any other philosophical system as part of their ascetic labors. While several Greek philosophers including the Neoplatonists formulated theories based on Trinitarian ideas, none of these was ever adopted into Orthodox usage as all are based on rational concepts of a divinity that can ultimately be understood by the human intellect, whereas Orthodox Christian theology is apophatic in nature; this means that although God is personally known and experienced through His divine energies, in His "essence" his incomprehensibility remains absolute. The soul knows God through mystical union, not philosophical speculation. Orthodox theology is the result of divine revelation exclusively. As Vladimir Lossky says, "All that can be said in regard to the platonism of the Fathers ... is limited to outward resemblances which do not go to the root of their teaching, and relate only to a vocabulary which was common to the age."
As the Christian becomes purified, in time he reaches the stage of theoria or illumination. At this point, the contemplative life begins. All ascetic practice must be understood as simply the means by which the goal of Christian life is pursued. This is the acquisition of the Holy Spirit, which is called theosis, meaning the "deification" of man. According to St. Athanasius and others, "God became man so that man can become god."