The it is Jesus' mission on this earth -- it was finished when Jesus died on the cross and gave up the ghost.
John 19:30 When Jesus therefore had received the vinegar, he said, It is finished: and he bowed his head, and gave up the ghost.
Of course, that means that Jesus is not coming BACK to this world, as Jesus said in other verses:
John 14:19 Yet a little while, and the world seeth me no more; but ye see me: because I live, ye shall live also.
John 17:11 And now I am no more in the world, but these are in the world, and I come to thee. Holy Father, keep through thine own name those whom thou hast given me, that they may be one, as we are.
John 17:4 I have glorified thee on the earth: I have finished the work which thou gavest me to do.
So the hundred-dollar question is why Christians believe that Jesus is coming back to this world.
The 50 cent answer:
Acts 1:11,
“Men of Galilee,” - - they said, “why do you stand here looking into the sky? This same Jesus, who has been taken from you into - heaven, - will come back in the same way - you have seen Him go into - heaven.”
Just because someone finishes their work doesn't mean they can't go back to the shop.
Then there's Philippians 3:20, 1 Thessalonians 1:10, 2 Thessalonians 1:10, Matthew 24, 30, and many others that say Jesus will return again.
1 Pet 1:10-11,
10 Of which salvation the prophets have enquired and searched diligently, who prophesied of the grace [that should come] unto you:
11 Searching what, or what manner of time the Spirit of Christ which was in them did signify, when it testified beforehand the sufferings of Christ, and the glory that should follow.
The prophets had some understanding that Jesus would actually make 2 different appearances, one as the sacrificial lamb and the other as King of Kings and Lord of Lords. What they couldn't figure out was what happens between the 2 advents. That would be our time which God kept secret until he revealed to Paul (Rom 16:25, Eph 1:9-11, Eph 3:4, et. al.).
Nobody in the OT knew about our age of grace and that Gentiles would be included with the Jews in God's grand scheme, not even the man Jesus Christ. That explains why he said that some of those to whom he spoke would be around for his second advent in Matt 16:28. He thought he'd reappear in 70 years as per Daniel. He could only know the scriptures that God had revealed up to his lifetime. Jesus did not know about the mystery. No man knew until God revealed it to Paul.
Why did God keep it a secret?
1Cor 2:8,
Which none of the princes of this world (Greek aion = age or era) knew: for had they known [it], they would not have crucified the Lord of glory.
Why would the devil not have killed Jesus? When Jesus was here the first time he could be in one place at one time doing one thing. That was bad enough for the devil, but when he killed him that made it possible for God to raise him from the dead which meant death was conquered. But the biggest coup was that because of Jesus' death and resurrection it is now possible to be born from above which among other things includes the truth that Christ resides in each and every single born again Christian (Col 1:27). Furthermore, Jesus said that all those Christians could do the things he did and even more (John 14:12). So now instead of one Jesus doing good works there are billions that can do even greater works. The devil shot himself in the foot! Of course, most Christians don't know about the mystery and Christ in them (the churches don't teach that plain truth in Bible), so that's why we don't see those greater works on a grand scale. I've seen them and so have others, but most have been blinded to that truth.
Well, I guess the 50 cent answer ended up being at least $1.50.
Take care