Since Satan was defeated by Jesus on the Cross, why is he still allowed to vex us today? In the Revelation it says that Satan was overcome by Salvation, and yet he continues to vex Christians today.

I know that at the great white throne judgment he will be cast into the lake of fire, but it seems that once we give our lives to Christ, and we have through the Grace of God, been plucked out of Satan's hands, that he would no longer have any power to trouble us.

Revelation 12:9-12  KJV  And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him. And I heard a loud voice saying in heaven, Now is come salvation, and strength, and the kingdom of our God, and the power of his Christ: for the accuser of our brethren is cast down, which accused them before our God day and night. And they overcame him by the blood of the Lamb, and by the word of their testimony; and they loved not their lives unto the death. Therefore rejoice, [ye] heavens, and ye that dwell in them. Woe to the inhabiters of the earth and of the sea! for the devil is come down unto you, having great wrath, because he knoweth that he hath but a short time.

Since the Christian has already rejected Satan and his ways and been given eternal life through Jesus, why does Satan still have the power to vex us?
One of the theological problems that has plagued theologians and Christians generally throughout the church age is theodicy.  

At the heart of theodicy is an apparent contradiction.  On the one hand you have an all-powerful, all-knowing, all-loving, and holy God, whose purpose concerning humankind is to redeem a people for His glory.  This people, composed of members of every ethnic group in the world, will one day bear the image of God's well beloved Son and our Savior, Jesus Christ, who came to earth to destroy the works of the devil (1 John 3:8).     

On the other hand you have a powerful, cunning, hate-filled, unholy spiritual being, originally created sinless by God, but who allowed pride to enter his heart and rebelled against God's authority.  It is Lucifer, a fallen angel, who is responsible not only for introducing sin into the universe but also for much of the misery on planet Earth.  His goal is to usurp God's Lordship in the universe and to dethrone God.  His modus operandi is to kill, steal, and destroy (John 10:10); in short, to do everything within his finite, though awesome, power, and for his own glory and honor, to defeat the works of God.  As Martin Luther said, 


  "His craft and power are great, And armed with cruel hate, On earth is not his equal" (A Mighty Fortress Is Our God). 


How, then, can we reconcile these two seemingly irreconcilable truths?  Can they in fact be reconciled?  I suggest the greatest existential problem which has confronted humankind from the Garden of Eden to today has in its solution the answer to these difficult and vexing questions. 

There are three phrases which together can frame an answer to your question about the seemingly unnecessary protraction of Satan's diabolical activities.  Each phrase begins with the letter C, and they are:


  
  the C osmic war
  the C ross of Christ
  the C hurch of God
  


The cosmic war between God and Satan, which rages on planet Earth, the focal point and stage for this epic struggle between good and evil will ultimately be won by God.  As you pointed out in your question, God's Salvation is in one sense a fait accompli, but more on that in the next point.  

God has not been silent since Satan first challenged His power in the rebellion he instigated in heaven.  In the counsels of eternity God had a plan, and when God has a plan, no one and nothing can prevent that plan from coming to fruition.  Victory is assured, but on God's terms, not ours.  

If we human beings and not God were all-powerful and in charge of the universe, we probably would have thought it wise to destroy Satan and his angels from the get-go, annihilating him and his cohorts by fiat, thus sparing humankind from the ravages of sin and death.  I know that's how I would have done things!  

Thank God, however, we are not God!  Our point of view is finite and flawed; God's point of view and plan are infinitely wise in both conception and implementation.  Moreover, not one of His attributes is compromised in any way as He brings His plan for the ages to fruition.  

What if God were to have annihilated Satan the very moment pride entered Lucifer's heart?  After all, He is all-powerful, is He not?  Yes, of course.  God is much more than the possessor of awesome power, however, and He most definitely is not a cosmic bully who zaps His enemies the very moment they begin to rebel.  God created sentient beings in the first place (angels first and then humankind) so they could enter into relationship with Him by their own choice.  He gave us all the ability to make decisions.  The prohibition God gave to our first parents would not make sense otherwise.  Why tell someone not to do something if they cannot do otherwise?  That's not freedom; that's something altogether different. 

In other words, God is all about relationships.  He did not create angels and human beings because He needed them; He did so in love, so they would have the privilege of glorifying and enjoying Him forever.  God also takes delight in His creatures, especially when they willingly and wholeheartedly love, worship, serve, and adore Him. 

Furthermore, God's delight in us extends to every child of His, each one of whom is unique and has something uniquely special to offer God.  That is true not only here on earth, but it will be especially true in heaven, where we will be distracted neither in our worship of God nor in our service for God.    

We must not take lightly the radical freedom we have on earth to choose to obey or disobey God. How we exercise that freedom determines our eternal destiny, and God desires that as many people as possible join His forever-family.  Sin, however, prevents this from happening.  Since we all are fallen creatures in need of God's forgiveness and cleansing, God provided a way whereby He could justly and righteously put away the sin that separates us from Him.  He did so by making the greatest sacrifice possible:  by giving us His well beloved Son, our Lord and Savior Jesus Christ.

The cross of Christ, then, was the means whereby holy God could atone for our sins, thus turning aside His righteous anger from us and onto Christ, "the Lamb of God who takes away the sin of the world."   


  "For it was fitting for Him, for whom are all things, and through whom are all things, in bringing many sons to glory, to perfect the author of their salvation through sufferings" (Hebrews 2:10 NAS). 


The cross of Christ, then, was fallen humanity's only hope.  The "perfection" of which the writer to the Hebrews speaks is not sinless perfection but "a fitness for the fulfilling of the office assigned to Him" (see Everett F. Harrison in “The Theology of the Epistle to the Hebrews,” Bibliotheca Sacra 121:484, October-December 1964:338).   In other words, Jesus was the high priest who offered Himself as a sacrifice for sin, once and for all, and through the unimaginable suffering He endured in being crucified, we have been redeemed--set free--through His shed blood. 

At the cross, Jesus wrestled with Satan and won.  Satan indeed had bruised Jesus' heel, but Jesus had dealt Satan a death blow (see Genesis 3:15), which brings us to the final phrase. 

The Church of God is a living organism composed of believers in Jesus, each one of whom has placed his or her faith in Him and received Him into their lives as Savior and Lord.  Through the Church of God, the Body of Christ, God demonstrates to the world and to Satan and his minions that God will prevail, but only after the church universal is complete and the last "living stone" is added to the superstructure of which Christ is the chief cornerstone (see 1 Peter 2:4-8).

After the church is gathered up to heaven (1 Thessalonians 4:16,17), the earth will undergo a cataclysm unlike any other it has experienced since the beginning of time.  Through the events of the Great Tribulation and the Battle of Armageddon spoken of in Revelation, the earth as we know it will be destroyed; Satan, the archenemy of God and of His saints, will be cast forever into the lake of fire, along with his minions; and the unbelieving portion of humanity who refused to repent of their sins will along with Satan be cast into the lake of fire.  God will then usher in a new heavens and a new earth in which righteousness dwells forever (see Revelation 20:7-21:8).  

In the meantime, the church of God has to endure much suffering.  If our God and Savior, our Lord Jesus Christ, the pioneer of our salvation had to be perfected through suffering--and he had committed no sin, can we as His redeemed people, whose sins were nailed to His cross, expect any less (see Colossians 2:14)?  Suffering is hard, but for those who endure it there are great rewards:  patience, endurance or perseverance, proven character, God's comfort to us and through us to others, and hope, because as Paul points out, 


  "and hope does not disappoint, because the love of God has been poured out within our hearts through the Holy Spirit who was given to us" (Romans 5:3-5).


In conclusion, God allows Satan to remain on the scene today, for a time, because God's timetable, though fixed, cannot be rushed.  Furthermore, God's forbearance and patience are not to be construed as slackness.  As Peter said, 


  "The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish , but that all should come to repentance" (2 Peter 3:9 KJV).


While for a time God will allow Satan to do his best to make life difficult for us, we can take comfort in Martin Luther's great hymn:


  And though this world, with devils filled, should threaten to undo us,
   We will not fear, for God hath willed His truth to triumph through us:
   The Prince of Darkness grim, we tremble not for him;
   His rage we can endure, for lo, his doom is sure,
   One little word shall fell him.
  
  That word above all earthly powers, no thanks to them, abideth;
   The Spirit and the gifts are ours through Him Who with us sideth:
   Let goods and kindred go, this mortal life also;
   The body they may kill: God’s truth abideth still,
   His kingdom is forever.


Another hymn which underscores the importance of endurance in running the race set before us, is Isaac Watt's "Am I a Soldier of the Cross?":


  Am I a soldier of the cross,
   A follower of the Lamb,
   And shall I fear to own His cause,
   Or blush to speak His Name?
  
  Must I be carried to the skies
   On flowery beds of ease,
   While others fought to win the prize,
   And sailed through bloody seas?
  
  Are there no foes for me to face?
   Must I not stem the flood?
   Is this vile world a friend to grace,
   To help me on to God?
  
  Sure I must fight if I would reign;
   Increase my courage, Lord.
   I’ll bear the toil, endure the pain,
   Supported by Thy Word.
  
  Thy saints in all this glorious war
   Shall conquer, though they die;
   They see the triumph from afar,
   By faith’s discerning eye.
  
  When that illustrious day shall rise,
   And all Thy armies shine
   In robes of victory through the skies,
   The glory shall be Thine.