Mortification of the flesh is a term used to describe a phenomenon which is common to almost all religions in the world. It entails putting one's body through some sort of discomfort in an attempt to attain divine salvation. It can include anything from denying one's body certain pleasures such as abstaining from alcohol, certain foods, sex to more harsher forms such as fasting, self flagellation etc.
The essence of this phenomenon in my opinion is to be able to pay attention to one's inner unrest beyond the obvious outer unrest. We are born with certain desires which Sigmund Freud referred to as Id. These desires are essentially for survival and include things like food, sex and shelter. These are common to all animals but I believe as humans became more cognitively sophisticated and started controlling their environments, their desires grew beyond just the basic survival needs. Now man started aspiring more from life: wealth, fame, recognition etc. Sometimes these desires take over one's life and start controlling him/her. One compromises his/her morals, principals and everything he/she stand for to earn more money, fame or recognition. Eventually this grows to the point that those desires become who you are and you are ready to beg, borrow, steal, lie, subdue, kill etc to achieve those desires. On most occasions this is very evident to everyone around that person but him/her. And this is where mortification of the flesh played a pivotal role historically. The very fact that it is common to almost all faiths, makes me believe that it has a very strong spiritual basis. This phenomenon allowed one to give up those desires and focus on something more deep, understanding one's own unrest and be at peace with yourself. In my opinion, the easiest way of describing this phenomenon is by the quote of Blaise Pascal: All of the man's problems stem from his inability to sit quietly in a room alone.
It was only time that man found a way around this, and here comes the practice part of this phenomenon. In my opinion, this phenomenon has been lavishly corrupted. Simple example: I have seen people fast and all they do is think about food. Even worse is the transactional nature of this phenomenon. People of all faiths use this phenomenon as a bribe to the divine power they identify themselves as. They put one's body through these 'inconveniences' and feel that they have done something huge for "God all mighty" to take notice and grant them their wishes. This is something which baffles me. If there is an existence of the so called God which resides in some heavenly place why would he/she/it notice you when there are many millions (if not billions) doing the same thing. I believe this corruption is the result of man's desperation for those desires. Lets look at how it started, I believe this practice is not the result of the entire human race but is the product of a few. The salesmen of God industry (priests) have utilized the ultimate power of God to keep masses scared. Scared masses are easy to control, they are a good source of revenue and being scared stops the logical and rational thought process, so they don't question the salesmen. Scared masses feel respite in a feeling that a divine force is watching over them and feel that it is their duty to protect the honor and respect of that divine force. Most of the wars in human history have been fought around this contrivance, millions of lives have been lost and billions of lives have been and currently are tormented by this.
All of the major religious texts talk about focusing inwards and have focused on the divine energy inside you and not in a book or a statue or a figurine. The divine salvation is when you are at peace with yourself. That is moksha.
Just my 2 cents.
Thanks for reading, I look forward to your comments!