Saint Simeon The New Theologian
A man given to argument becomes his own double-edged sword; he destroys his soul, without knowing it, alienating it to the Kingdom of Heaven. An argumentative man deliberately gives himself to his king's enemies. His argument is a fishing line baited with some veracity -- defense of truth, self-justification, self-defense -- that reels him into swallowing the hook of sin. Then evil spirits can ravish his poor soul, hooked both by tongue and throat. Now rising upwards, now sinking in the chaotic abyss of sin, his argumentative soul is condemned, with those souls cast down from heaven.
A man who is deeply wounded in his heart by provocative and abusive argument harbors deep inside himself the old serpent of sin. If he bears the blows of argument in silence or answers with great humility, he will render this serpent powerless (or he will kill it altogether). But if he argues with bitterness or speaks with arrogance, he will give the serpent added strength to pour more poison into his heart and to devour his entrails mercilessly. By gaining strength daily, the serpent will finally eat away the intention and capability of the poor man's soul to mend its way of life. Thereafter, the man will live for sin and be deadened to the truth
With prayers and tears, implore God to give you a saintly instructor, free from passions. Study also the Holy Scriptures, especially the practical writings of the Holy Fathers, in order to compare them with what you are being taught by your teacher and preceptor. Thus will you see, as in a mirror, how far they agree. Keep in your thoughts what corresponds to the holy writings. And after wise deliberation, put aside what does not correspond, lest you fall into sin.
Increasing knowledge of God lessens interest in all else. The more a man knows of God, the less he cares for other matters. And he begins to know more and more clearly that he knows so little of God. The more radiantly God shines in such a man's spirit, the more He becomes invisible, and the more a man's sense soars above his senses, the less he needs to sense externals.
Saint Simeon The New Theologian.