Our old self was crucified with Him

Numbers 33:55-56 (ESV)
But if you do not drive out the inhabitants of the land from before you, then those of them whom you let remain shall be as barbs in your eyes and thorns in your sides, and they shall trouble you in the land where you dwell. [56] And I will do to you as I thought to do to them.

Those of them whom you let remain shall be barbs in your eyes and thorns in your sides. God gives warning here to Israel that if they do not destroy or expel the wicked inhabitants in the land, that they will become trouble. Israel were not to have pity on them, nor make them to remain in the land that they may do business with them or enter into covenants with them. They were not to make friends with them, or to intermarry with them, not to associate with them at all. They were to be destroyed by the edge of the sword.

We know that Israel did not obey the Lord in this matter, and the inhabitants were not completely driven out of the land. And the remnants of this evil seed influenced Israel to commit sins, and ultimately Israelites themselves were cast out of the land, invaded by Babylon.

Let us Christians likewise be warned that little leaven can leaven the whole lump. We are to cast out any remnants of sin that remain within us. We do wisely to be warned that it was these little thorns and barbs in Israel that became like infections that eventually destroyed them whole. Likewise it is the small besetting sins that we carry from our old lives that can destroy and make our faith a shipwreck. We are to utterly purge out the old leaven that is within us:
1 Corinthians 5:7-8 (ESV)
Cleanse out the old leaven that you may be a new lump, as you really are unleavened. For Christ, our Passover lamb, has been sacrificed. [8] Let us therefore celebrate the festival, not with the old leaven, the leaven of malice and evil, but with the unleavened bread of sincerity and truth.
Paul says here we are to be as unleavened bread. Unleavened is the absence of leaven. There must therefore be a total absence of malice and evil in us. Only a LITTLE yeast is required to make the dough expand. Likewise, only little evil is required to destroy the Christian. Let us be warned against little sins. Little sins are enough to trouble us for life. Let us be sensitive to the little blemishes and spots, that we may cleanse them out with repentance and faith in Christ.

We are to cast off from us our old self from within us, just as Israel cast out the old inhabitants from the land:
Ephesians 4:21-24 (ESV)
assuming that you have heard about him and were taught in him, as the truth is in Jesus, [22] to put off your old self, which belongs to your former manner of life and is corrupt through deceitful desires, [23] and to be renewed in the spirit of your minds, [24] and to put on the new self, created after the likeness of God in true righteousness and holiness.

Our old self is the identity that we had in Adam before we were born again by God and made a new creation in Christ. It was the self that loved and savoured earthly things and not the things of God. It was the self that always thought, spoke and did that which was hostile and evil in the eyes of God. This was who we were, before we were made new creatures and given new selves, which were born of the Holy Spirit, and made after the image and likeness of Jesus Christ. God had given us a new mind and new heart which loves and agrees with the law of God. God had given us a heavenly self, which is capable of thinking of heavenly things. God made us, his enemies, to be His children by the Spirit of adoption, pouring upon us the love that was upon Christ.

But like the Canaanites that lived in the land after Israel had invaded the land, like parasites that live off the host, our old selves remain in us throughout our lives. Relics of sin dwell in us, making us sometimes to walk in these evil things we used to walk. Therefore it is our continual duty to eradicate the earthly man with his deeds. We won't be able to destroy him in one day, as the Canaanites were not defeated in one day, but only little by little. Neither will we destroy all of him in our while we live, and some sins may remain within us until old age, finally to be done away with when we enter into glory. Hence it is our job to continually weaken and put him to death. Here are the list of things which belonged to our Adamic self, which we must kill:
Colossians 3:5-11 (ESV)
Put to death therefore what is earthly in you; sexual immorality, impurity, passion, evil desire, and covetousness, which is idolatry. [6] On account of these the wrath of God is coming. [7] In these you too once walked, when you were living in them. [8] But now you must put them all away: anger, wrath, malice, slander, and obscene talk from your mouth. [9] Do not lie to one another, seeing that you have put off the old self with its practices [10] and have put on the new self, which is being renewed in knowledge after the image of its creator.

But how do we put to death the old man? Is it by making our behavior and body to be suppressed by rules and regulations? Is it by following the different man-made "steps" or "guides", that our actions become more refined? Will that stop the old man within us from raging against the Holy Spirit? Following a bunch of rules may reprogram our outward actions, but it cannot kill the lust within us, as it is written:
Colossians 2:20-23 (ESV)
If with Christ you died to the elemental spirits of the world, why, as if you were still alive in the world, do you submit to regulations- [21] "Do not handle, Do not taste, Do not touch" [22] (referring to things that all perish as they are used)-according to human precepts and teachings? [23] These have indeed an appearance of wisdom in promoting self-made religion and asceticism and severity to the body, but they are of no value in stopping the indulgence of the flesh.

Nothing outward can ever make us right in God's eyes. If inwardly we are corrupt, no matter how much we may outwardly we do acts of good, we will never fool God, for:
1 Samuel 16:7 (ESV)
But the LORD said to Samuel, "Do not look on his appearance or on the height of his stature, because I have rejected him. For the LORD sees not as man sees: man looks on the outward appearance, but the LORD looks on the heart."
This is why scripture says that no FLESH shall ever be justified in His sight, for God looks at the heart, the spirit of the man. If the spirit of the man is dead and corrupt in sin, no matter how good the outward appearance with its actions may be, the man shall never be justified in God's sight. For God sees the man's heart, and not only the outward:
Romans 3:20 (KJV)
Therefore by the deeds of the law there shall no FLESH be justified in his sight: for by the law is the knowledge of sin.

The old self can never be contained by outward actions. Our old self can never be reconciled with God, nor inherit the Kingdom of God. It cannot be modified or changed at all. It will always be hostile to God, and never submit itself to God's laws. There is no hope in our natural earthly self in ever pleasing God. It must die. It must be inwardly & spiritually destroyed. And this is done by believing in Jesus Christ and His Gospel:
Romans 6:6 (ESV)
We know that our old self was crucified with him in order that the body of sin might be brought to nothing, so that we would no longer be enslaved to sin.
It is by believing that our old self in Adam died with Him on the day of Calvary. It is by believing that Christ killed him when He condemned sin in His own flesh, when He died as our representative. When we became born again, we were spiritually united with Christ in His death. The day that our new self was born, our old self was put to death on the cross with Him. Therefore bible tells us to consider ourselves as dead. This is how we kill the old man:
Romans 6:9-11 (ESV)
We know that Christ, being raised from the dead, will never die again; death no longer has dominion over him. [10] For the death he died he died to sin, once for all, but the life he lives he lives to God. [11] So you also must consider yourselves dead to sin and alive to God in Christ Jesus.

This "considering" ourselves to be dead can be compared to this verse:
Luke 22:37 (ESV)
For I tell you that this Scripture must be fulfilled in me: ''And he was numbered with the transgressors.'' For what is written about me has its fulfillment."
The word "numbered" is the same word used in Romans 6:11 "consider" or "reckon". Therefore, just as Jesus Christ was considered to be a sinner, though He was not, we are to consider ourselves as dead, though we live. Just as our faith in Christ was "reckoned" by God to be righteousness, we are to likewise reckon ourselves as dead. Just as righteousness was "imputed" (same Greek word) to us, though we are not righteous, we are to impute death to ourselves, though we are alive to God in Christ. See how Paul considered himself to be dead, though he was living:
Galatians 2:20 (ESV)
I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me.

Mortification is perplexing because in one sense, we are told that we are already dead with Christ: Paul says that we "have died and our life is hidden with Christ" (Colossians 3:3). It is like a corpse, a decomposing body of death, ready to be perished forever. It has been buried in Christ, as a seed in good soil. However, in another sense, our old man is still very much alive in us. We feel its motions. Like the remaining inhabitants of Canaan, it still dwells in us, and fights against us, plotting against us. Though it is meant to be dead, it still rebels and wages war against the Holy Spirit in us. It foams in its mouth frantically, desiring to reign over us again.

However, our perplexities are relieved when we consider the picture of crucifixion. A crucified man is in a sense already dead. There can nothing be done to deliver him from his state. The more he moves his body to save himself, the more pain and destruction will be done to his body. No one can take down an already crucified man. Death by crucifixion is not like dying by a disease, in which one may even have a chance to be healed. But the crucified man is definitely bound for death, and thus can be reckoned to be already a dead man. However, he is still alive on the cross, for hours, sometimes even for days. It is a long, drawn out dying process. The man, while on the cross, may consider himself to be alive, and being deliberately ignorant of his crucified state, may say and do things within the confines of the cross. He may yell and shout, or jerk himself violently in pain. The man may even deceive himself that he is NOT crucified, and that he is fully alive. But when he looks to his left and right palms, and down to his feet, reality will come straight back to him, to remind him that he is actually dying. This is what it means to consider ourselves as dead. This is the picture of our mortification.

Let us therefore have nothing to do with our old self, as much as the Israelites had nothing to do with the inhabitants of Canaan. We cannot be friends with it, nor make a covenant with it, nor be friends at all with it. Just as Israelites had nothing to do with touching dead things, we also must stay away from our old selves, lest it defile us. We must let it stay in the tomb where Christ lay, and roll the stone over it that it may never come out. Unless we do this, our old selves will become as barbs and thorns that will kill us. Yes, it is hard work, to constantly consider ourselves to be dead, to always deny our old self any pleasures, to always discourage it. It hurts to give our old self vinegar when it desires water for its thirst. There is suffering involved. But there is no other way for the Christian to live. This is the cross that we ought to bear, if we want to follow Christ wherever He goes.

And I will do to you, as I thought to do to them. This is solemn warning, which came true for Israel. Just as God destroyed the Canaanites by Israel, God destroyed Israel by other nations. We must thus also be warned as Christians. Not that God can destroy His own child after He has saved him. Not that God's faithfulness can fail completely. But if we do not repent, or we don't consider ourselves as dead in Christ and continue doing in the deeds of the old self, then God will chastise us for our sins. If we do continue to live to sin and not to God, then God will make us to partake in the judgment of sinners, at least in part. This is because He loves us as a Father.

Let us remember the punishment that Christ took for the sins of our old self. It was because of our sins that Christ was beaten, whipped, spat on and accursed. It was because of our evil deeds that He was put to shame. All that wrath fell upon Him, and not upon us, because God spared us, by His own grace. Can we then continue in sin? If we continue in sin, and we abide not in Christ, will not God's wrath fall upon us? But we know that those whom Christ has chosen will not be forsaken and be left to such fate. But let us consider such harsh warnings of God as grace, and do the work of mortification by faith in Christ. God will do this work in us, by His Holy Spirit. Amen.