“There is no cleanness in him who thinks evil,” says St. Simeon the New Theologian; and he further adds, “How can there be a pure heart in him, who soils it with impure thoughts as a mirror is darkened by dust?” Do you see then, the inexcessible height of which the religion of Christ stands above all other
faiths and worldly sophistries? He who only thinks about evil, even though he does not commit any evil, is at fault before God and before his own soul. For he offends God and loses his soul.
To be a Christian, in the proper sense, means to invest enormous effort on cleansing evil thoughts from one’s heart and mind. What kind of effort is that? There exists a complete study about this, which in our days
became completely locked away even for us Christians, and the one enormous actual experience of holy men and women who justified that study. To cleanse oneself from these wicked and impure thoughts, the root of all evil, was the goal of all the great ascetics, hermits and silentaries.
Saint Nikolai of Ochrid