The “flesh” is all the sinful desires/temptations/lusts, etc. that live in human nature. It is the source of temptation, and there lives nothing good in the flesh. (Galatians 5:19-21; Romans 7:18 ; Galatians 5:24; Romans 8:5.) In other Bible translations the words “sinful human nature”, “sinful nature” etc. are used instead of “flesh”.
Sin is anything that goes against God’s will and His laws. To commit or do sin is to disobey these laws. The desire to sin lives in human nature. In other words, human nature is made corrupt and badly influenced by the sinful habits and desires that live in all people as a result of the fall into sin and disobedience in the garden of Eden. This is the “body of sin” (Romans 6:6) that we are all born with. The Bible also calls these sinful habits and desires the “lusts of the flesh” or “desires of the flesh”. This means that as human beings we are tempted by sinful desires and thoughts from our own sinful human nature. John writes that we all “have sin”, but you are not guilty of doing sin unless you first knowingly agree to the sinful desires. Sin can be “put to death” bit by bit by not giving in to these lusts and desires as you start seeing them as being sin. This process first happened in Jesus while He Himself was a human being on earth. (1 John 1:8; Romans 6:6; Romans 7:18; Romans 8:3-4.)
The term “flesh” can also mean humankind, or our physical bodies, especially in the Old Testament. (Genesis 6:13; Psalm 56:4; 1 Peter 1:24; Ephesians 5:29.)