Jesus says, "I have overcome the world."

33 I have said these things to you, that in me you may have peace. In the world you will have tribulation. But take heart; I have overcome the world.

Jesus promised his disciples peace before his crucifixion, he says, “Peace I leave with you; my peace I give to you.”(John14:27) This is the peace that is not conceived by the flesh but by faith in his word. Jesus came and took our flesh, suffered, died and rose from the dead to ensure that this peace is earned for us. Jesus ascended into heaven and then send the promised Holy Spirit who reveals to us this eternal peace. It is through Jesus’ passion, resurrection and ascension that our sins are forgiven.

The peace that Jesus talks about is not the peace with the world but it is the peace with Father. Remember when the diabolic one came into the dwelling place of man in the Garden of Eden, man disobeyed God by listening to the twisting word of the devil. “Did God actually say..” (Genesis 3:1)Then, man fell into sin and there was dissension between God and man.

Jesus is strengthening the souls of the disciples, encouraging them to continue in the faith, and saying that through many tribulations we must enter the kingdom of God.  

He is encouraging them because he knows that they will face sufferings because of their faith in him.