Before the cross, Satan roamed freely in the world and had great power over mankind.

"Now there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them.  And the LORD said unto Satan, Whence comest thou? Then Satan answered the LORD, and said, From going to and fro in the earth, and from walking up and down in it."    Job 1:6-7

"And the LORD said unto Satan, Behold, all that he hath is in thy power;"
Job 1:12

Prior to His death, Jesus had just finished preaching as the perfect preaching for three and a half years, and yet there were very few people who had become saved. This is evident by the account in Matthew chapter 27 where Jesus is standing before Pontius Pilate. It was customary then, on the day before the Passover, for the governor to release one of his prisoners. When Pilate asked the crowd which prisoner they wanted released, they all asked for Barabbas. 

"The governor answered and said unto them, Whether of the twain will ye that I release unto you? They said, Barabbas. Pilate saith unto them, What shall I do then with Jesus which is called Christ? They all say unto him, Let him be crucified. And the governor said, Why, what evil hath he done? But they cried out the more, saying, Let him be crucified."
Matthew 27:21-23 

It was not until after Christ's death on the cross, when God was able to bind Satan. In Mark 3 we see that Jesus refers to Satan as "the strong man". God had to bind the strong man first, in order to enter into his house and plunder it. 

"And if Satan rise up against himself, and be divided, he cannot stand, but hath an end. No man can enter into a strong man's house, and spoil his goods, except he will first bind the strong man; and then he will spoil his house."
Mark 3:26 

In Revelation 12 we read an account describing a battle in heaven which is occurring between the Lord Jesus, represented by Michael, and Satan. After this battle, God is victorious in casting Satan down to the earth from heaven. We know this occurs after Christ's death on the cross because of the statement in verse 11.

"And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels, And prevailed not; neither was their place found any more in heaven. 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."
Revelation 12 7:11

At the cross God bound Satan for a period of time which the book of Revelation describes as 1000 years. God gave Satan a "death blow", also described as a "deadly wound". In the Garden of Eden, God told Satan that the seed of the woman would “bruise his head”. 

"And the LORD God said unto the serpent, Because thou hast done this, thou art cursed above all cattle, and above every beast of the field; upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life: And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel."
Genesis 3:14-15 

Before the cross, Satan had great authority over mankind and that is why Jesus called him the strong man”. It is at the cross that Satan received a “death blow”. It is at the cross that the seed of the woman “bruised” the head of the serpent. It is at the cross that God, with His "keys to hell and with a great chain in his hand," cast the dragon into the bottomless pit and, set a seal over him, bound him for 1000 years. 

"And I saw an angel come down from heaven, having the key of the bottomless pit and a great chain in his hand. And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years, And cast him into the bottomless pit, and shut him up, and set a seal upon him, that he should deceive the nations no more, till the thousand years should be fulfilled:"
Revelation 20:2-3 

"I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell and of death."
Revelation 1:18 

God bound Satan for the sole purpose of preventing him from deceiving the peoples of the world. God had to bind Satan at the cross because it was after the death of Jesus that the Holy Spirit would be poured out. It was through the Holy Spirit that God began the work of evangelizing the world and building up the New Testament church. In Acts chapter one, a short time after Jesus had ascended into heaven, we find the apostle Paul still promising people that the Holy Spirit would soon be coming. 

"The former treatise have I made, O Theophilus, of all that Jesus began both to do and teach, Until the day in which he was taken up, after that he through the Holy Ghost had given commandments unto the apostles whom he had chosen: To whom also he shewed himself alive after his passion by many infallible proofs, being seen of them forty days, and speaking of the things pertaining to the kingdom of God: And, being assembled together with them, commanded them that they should not depart from Jerusalem, but wait for the promise of the Father, which, saith he, ye have heard of me. For John truly baptized with water; but ye shall be baptized with the Holy Ghost not many days hence."
Acts 1:1-5 

Soon after, God poured out His Holy Spirit on the first day of Pentecost, and several days later after preaching for one day, the apostle Paul was able to bring three thousand souls into the Kingdom of Christ. 

"And with many other words did he testify and exhort, saying, Save yourselves from this untoward generation. Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls."
Acts 2:40-41 

"And when the day of Pentecost was fully come, they were all with one accord in one place. And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting. And there appeared unto them cloven tongues like as of fire, and it sat upon each of them. And they were all filled with the Holy Ghost."
Acts 2:1-3 

The Bible states that after 1000 years has expired, God looses Satan from his prison, or the bottomless pit. 

"And when the thousand years are expired, Satan shall be loosed out of his prison, And shall go out to deceive the nations which are in the four quarters of the earth, Gog, and Magog, to gather them together to battle."
Revelation 20:7-8

If Satan is loosed 1000 years after he was bound at the cross, doesn't that mean he would have been set free around the year 1000 A.D.?  When we read the Bible, we must remember that God often times speaks in parables. 

"And with many such parables spake he the word unto them, as they were able to hear it. But without a parable spake he not unto them."
Mark 4:33-34 

All numbers in the Bible signify something. For example, the number 2 signifies the church; the number 3 signifies the purpose of God; the number 4 signifies universality; the number 5 signifies God’s grace or judgment; the number 6 signifies work; the number 7 signifies perfection or totality; the number 8 signifies redemption or salvation; the number 10 signifies the completeness of whatever is in view; and there are other numbers. When we take the number 10 and multiply it by 10 and then by 10 again, we have a perfect cube and the number 1000. Since the number 10 signifies the completeness of whatever is in view, when we multiply the number 10 by 10 twice,  we can interpret that to mean that God is emphasizing the total completeness of whatever is in view. The expression “1000 years”, therefore, can be understood to represents the absolute completeness of time that God will keep Satan bound, while He builds up the New Testament church. This type of mathematics can be Biblically validated when we study the passage in Matthew 18:21. 

"Then came Peter to him, and said, Lord, how oft shall my brother sin against me, and I forgive him? till seven times? Jesus saith unto him, I say not unto thee, Until seven times: but, Until seventy times seven."     Matthew 18:21-22

Jesus did not have in mind that after we forgive someone 490 times we then stop at 491.   But rather that we are to forgive our brother 7 X 7 X 10.   That would signify that we forgive our brother a perfectly, total, and complete amount of times.