Romans 6:10 For in that he died, he died unto sin once: but in that He liveth, He liveth unto God. 11 Likewise reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord. 12 Let not sin therefore reign in your mortal body, that ye should obey it in the lusts thereof.

One of the major battle in life is detecting the enemy within. It is easy to recognize, spot, see, pick the enemies from outside. The most difficult battle to fight and win is fighting the enemy within, the reason is, this enemy sleeps and wake up with you.

Colossians 3:5 Therefore put to death your members which are on the earth: fornication, uncleanness, passion, evil desire, and covetousness, which is idolatry.

Fighting terrorism is difficult, because, while it is easy for nation to fight nations, a terrorist may be a neighbor, going to the same market with you wearing explosive. Until the bomb is detonated, you won't know that there is something on them. This has made it very difficult to fight the war against terrorism.

So also, it is difficult to fight the flesh. While you are aware of some things, there are others you don't even know until it spring up in the face of situation. Even though sin has been put to death, sometimes, the ugly smell of the old dead man try to slip out thus making our new life look like fake. Paul therefore admonished us to "reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord. Let not sin therefore reign in your mortal body, that ye should obey it in the lusts.