“What then is meant by the worship of God?
It means that we have nothing extraneous in our intellect when we are praying to Him: neither sensual pleasures as we bless Him, nor malice as we sing His praises, nor hatred as we exalt Him, nor jealousy to hinder us as we speak to Him and call Him to mind.
For all these things are full of darkness; they are a wall imprisoning our wretched soul, and if the soul has them in itself it cannot worship God with purity. They obstruct its ascent and prevent it from meeting God; they hinder it from blessing Him inwardly and praying to Him with sweetness of heart, and so receiving His illumination.
As a result the intellect is always shrouded in darkness and cannot advance in holiness, because it does not make the effort to uproot these thoughts by means of spiritual knowledge.”
-St Isaiah the Solitary, taken from the Philokalia