Christians have two natures. We all have the sin nature. The bible refers to this nature as the flesh. It is our human nature. That nature that is opposed to God and His laws. It is the nature that is drawn to sin. That carnal nature wants to sin. On the other hand, we have the spirit nature. This is the part in us that wants to obey and honor God. This is the nature that resists sin.
We need to mortify the flesh. The word mortify means to subdue and deny self. You can only mortify the flesh by resisting, saying no, and refusing to give into temptations. Temptations of the enemy are enticing. You could have said no to sin 1,000 times in a row, but the flesh is relentless and can trip you up on the 1001st times. Mortifying the flesh means crucifying the flesh. Read Galatians 2:20. Paul said he had been crucified with Christ. He was not talking in the literal sense. Symbolically and spiritually Paul advocated crucifying the flesh. That means dying to the flesh. I promise our flesh will not give up a single bit without a life and death struggle. Go ahead and test this. Choose an area of your flesh you want to crucify. Pay close attention how that area of your life will refuse to surrender to the cross of Christ.
If I decided I wanted to physically crucify my body, and grabbed hammer and nails I would not be able to complete the task. I could painfully get the nail in my feet and in one hand. What about the other hand? How could I nail the other hand to the cross if I already had a hand nailed? Somebody would have to do it for me. In the same way, you can't just decide to mortify the flesh, or put another way, to crucify the flesh. God has to do that in us. He has to mortify that carnal nature. It will resist, rebel, and refuse to submit. It is a supernatural work of God to mortify the flesh. THE FLESH NEVER GIVES UP.
Mortifying the flesh is not a one time decision. It is a daily decision. Maybe even an hourly one. You have to choose the path of surrender to God. He makes the spirit nature more dominant and subdues the sin nature. As long as we live in these bodies, we are going to fight against the sin nature. It is not impossible to overcome. It is just impossible to overcome in our ability.
Mortifying the flesh is not pleasant. It is painful. To say no to things your flesh wants to say yest to is excruciatingly difficult. You will be going against every natural desire, craving, yearning, and longing to do evil. Even some natural things that not bad. Try fasting from food. Try abstaining from television or social media. Try sleeping less so you can pray more. You will find yourself in a war. When the flesh is mortified, the battles are easier. You say yes to God and no to sin. It is that simple, when we mortify the flesh repeatedly.