Sin is a terrible master and it finds a willing servant in the human body. The body itself is not sinful , it is neutral. It can be controlled by sin or by God. But man’s fallen nature , which is not changed at conversion, gives sin a foothold from which it can attack and then control. Paul clearly expressed this problem when he said in
Romans 7:18 (NKJV)
18 For I know that in me (that is, in my flesh) nothing good dwells; for to will is present with me, but how to perform what is good I do not find.
A tremendous fact is introduced here: The old man( the old ego, self) was crucified with Christ so that the body need not be controlled by sin. The phrase “ that the body of sin might be done away with” means rendered inactive, made of no effect.