In my opinion the entire Orthodox doctrine of warfare against the passions finds an excellent jusification in the words of Christ Himself when he was speaking to the Pharisees. In Mark 7:14-15 we can read:
He [Jesus] summoned the crowed again and said to them, “Hear me, all of you, and understand. Nothing that enters one from the outside can defile that person; but the things that come out from within are what defile.”
Now, in context here Christ is speaking against the Pharisees’ strict legalistic and external practices, and in the verses above He is referencing the idea of unclean foods. He is telling the Pharisees that external sources, such as food as they believed, are not what make a person unclean in the eyes of God. Rather, it is what comes from within the person; how they act, what they say, what they prioritize, their feelings, thoughts, beliefs, and opinions.
This is why the Orthodox Christian is called to a life of struggle and deprivation. We are called to struggle in this lifetime and deprive our passions the excesses they desire so that we can purify ourselves for the life to come. In Russian the word for a spiritual struggle as such is ‘podvig,’ in Greek it is ‘askesis.’
Metropolitan Anthony of the ROCOR put it thus:
Christianity is an ascetic religion, Christianity is a teaching about the gradual extirpation of the passions, about the means and conditions of the gradual acquisition of virtues; these conditions are internal, consisting of podvig, and given from without, consisting of our dogmatic beliefs and grace-giving sacraments which have only one purpose: to heal human sinfulness and lead us to perfection.
So, let’s strive to throw off our passions, to heal our souls, to take up our cross and struggle for Christ. Let us fight for what we want instead of naively thinking that the prize will just be handed to us. Let us stop blaming outside sources for our failings and recognize that before we can live for our Almighty King in the manner which He requests, we must die to ourselves.