Our heart is incomplex, single, and therefore cannot "serve two masters: God and mammon" – that is, riches. This means that it is impossible to serve God truly and at the same time to be attached to earthly things, for all such things relate to mammon. Besides, it is unworthy of a man to serve riches, for they are earth and dust. All earthly things, if our heart attaches itself to them, make it gross and earthly, turn us away from God, from the Mother of God, and all the saints, from everything spiritual, heavenly, and eternal, and from love for our neighbour, and bind us to that which is earthly, perishable, and temporal. To complete that which has been said, it must be added that the spirit of attachment to earthly things, of sparing and grudging earthly things, is the spirit of the Devil, and the Devil himself dwells in the man through his attachment to earthly things. He often enters into our heart as an insolent conqueror, through some momentary attachment to earthly things, not immediately renounced, darkening, crushing, and deadening our soul, and making it incapable of any work for God, infecting it with pride, blasphemy, murmuring, contempt for holy things and its neighbour, opposition, despondency, despair, and malice.
Sergieff, Archpriest John Iliytch; St John of Kronstadt (2010-05-26). My Life in Christ, or Moments of Spiritual Serenity and Contemplation, of Reverent Feeling, of Earnest Self-Amendment, and of Peace in God (Kindle Locations 5206-5215). . Kindle Edition.