“Lay not up for yourselves treasures on the earth, where rust and moth corrupt and where thieves break through and steal.” Matt 6:19
For whenever we do anything with a view to human glory we know that we are, as the Lord says, laying up for ourselves treasure on earth, and that consequently being as it were hidden in the ground and buried in the earth it must be destroyed by sundry demons or consumed by the biting rust of vain glory, or devoured by the moths of pride so as to contribute nothing to the use and profits of the man who has hidden it.
We should then constantly search all the inner chambers of our hearts, and trace out the footsteps of whatever enters into them with the closest investigation lest haply some beast, if I may say so, relating to the understanding, either lion or dragon, passing through has furtively left the dangerous marks of his track, which will show to others the way of access into the secret recesses of the heart, owing to a carelessness about our thoughts.
And so daily and hourly turning up the ground of our heart with the gospel plough, i.e., the constant recollection of the Lord’s cross, we shall manage to stamp out or extirpate from our hearts the lairs of noxious beasts and the lurking places of poisonous serpents.
Cassian’s Conferences, 1st Conference of Abbot Moses, Chapter 22, NPNF2 11, p.306