Do you know what the mortification of sin is? Joshua 21-24

Joshua 23:12-13 (NASBStr)

For if you ever go back and cling to the rest of these nations, these which remain among you, and intermarry with them, so that you associate with them and they with you, know with certainty that the Lord your God will not continue to drive these nations out from before you; but they will be a snare and a trap to you, and a whip on your sides and thorns in your eyes, until you perish from off this good land which the Lord your God has given you.

So, what happens if we don't serve God and rid ourselves of the evil around us?   Israel was lead into the promise land by Joshua to take possession of the land.  God had given them victory on every side and had fought for them.  They were to simply become the caretakers of the land as God removed the old, evil caretakers.   This was not to be something they did, but God did through them.   All they had to do was drive out the inhabitants of the land.   In the above passage we read Joshua's words to the nation, however, if they DON'T drive out the inheritance.  If they didn't drive them out it wasn't going to be a neutral zone.   The enemy would eventually gain ground and conquer them, rather than be conquered by them.    The lesson for us is easy.   When we fail to drive sin out of our lives it isn't simply content to live along side us and become non-passive.   Sin is an aggressive body.   It attacks, always.   When we fail to deal with it in our lives it gains ground, immediately.   This is why we have to fight sin as early as possible when it enters the battle.   In the first thought of the mind we are in hand-to-hand combat with sin.   The first glance of the eye is simply sin in our body taking captive the tools of our body to do its bidding.   We must fight it then.   If we don't we allow sin to take advantage of our members (tools: eyes, thoughts, ears, senses) and use them against us.   If we don't conquer sin through the strength of The Lord than sin will conquer us.   We can't be happy to live in harmony with sin.   We must declare war on sin and attack it.  Paul called this the mortification of the body.  Note the following:

Romans 8:13 (NASBStr)

... for if you are living according to the flesh, you must die; but if by the Spirit you are putting to death the deeds of the body, you will live.

The "putting to death the deeds of the body" is the spiritual act of mortification.   Joshua is telling the Israelites the same thing. If they put away the evil around them they will live.  If not, death will begin to take over the body.